Angular rate sensor

ABSTRACT

An angular rate sensor comprises a piezoelectric film having a first and a second surfaces forming an x-y plane and utilizes a perturbation mass coherently vibrating elastic acoustic waves on which a Coriolis force acts when the angular rate sensor undergoes a rotary motion about an x-direction. A first elastic acoustic wave is excited in the piezoelectric film by a driving transducer and a second elastic acoustic wave generated by the Coriolis force proportional to an angular rate of the rotary motion of the angular rate sensor itself is detected by the detecting transducer. The angular rate sensor further comprises at least a first electrode disposed on the first surface of the piezoelectric film for discharging a surface charge caused due to piezoelectric effect at the lower surface of the film in which the first elastic acoustic wave is excited.

CROSS REFERENCE TO RELATED APPLICATION

The present application relates to and incorporates by referencesJapanese Patent Application Nos. 2006-202400 filed on Jul. 25, 2006,2006-202399 filed on Jul. 25, 2006, and 2006-202401 filed on Jul. 25,2006.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to angular rate sensors or gyroscopes, andmore particularly to angular rate sensors configured to measure the rateof an angular rotation in a method for detecting a magnitude of aCoriolis force generated by interactions between a vibrating motion of amass on a piezoelectric substrate and a rotary motion of thepiezoelectric substrate by converting the Coriolis force into a voltagedue to a piezoelectric effect.

2. Description of the Prior Art

There has been increasing demand for requiring angular rate information,which is also sometimes called angular velocity information not only inthe field of automotive products where, for example, inertial navigationand guidance systems, air-bug systems, and anti-skid systems use angularrate information for running, but also in other fields, for example, inthe field of still or video cameras where a manual blur correctingsystem can be found also in the field of medical products, surgicaltools, and body movement monitoring apparatus, all of these requireangular rate information. Generally a measuring device for obtainingangular rate information is called a gyroscope, gyro sensor, angularrate sensor, angular velocity sensor and so on. Recently, an inexpensiveoscillatory type angular rate sensor has been developed in order to beused in the above mentioned fields.

There has been proposed in recent years an angular rate sensor utilizinga piezoelectric element or substrate made of quartz or lithium tantalitecapable of providing a smaller and less expensive angular rate sensor.

One known prior art embodiment of an angular rate sensor utilizing asingle crystalline piezoelectric element typically has a pair of armswhich are joined and fixed at their individual end portions by a rootmember to form a tuning folk oscillator, as shown FIG. 37. Such anangular rate sensor is disclosed by, for example, U.S. Pat. No.5,719,460. Although a angular rate sensor disclosed in U.S. Pat. No.5,719,460 has more complex structures, a basic operation principle is asfollows. A set of drive electrodes are affixed to one of the arms of thetuning folk oscillator for driving the tuning folk oscillator in adirection of a principal plane at a resonant frequency due topiezoelectric effect which convert electric energy into mechanicaldeformation energy and vice versa. Thus, the driving electrodes areelectrically driven by an external oscillator circuit. A monitoringelectrode, and a sensing electrode are affixed to the other arm. Themonitoring electrode serves to detect the oscillation amplitudegenerated by the oscillator circuit. The sensing electrode serves todetect the stress caused by the Coriolis force acting on the tuning folkand being generated by a rotating motion of the angular rate sensor. Inmore detail, in order to keep the oscillation amplitude of the tuningfolk constant, the electric charge generated on the monitor electrodedue to the piezoelectric effect in the direction of the principal planeis amplified by the external circuit and then compared with a referencesignal to control the oscillator circuit. On the other hand, the sensingelectrode detects a signal generated by the Coriolis force, which isamplified synchronously with the signal detected by the monitoringelectrode.

Angular rate sensors of this type suffer from an inherent performancelimitation. For example, quartz is typically single crystallinepiezoelectric material composed of arrayed single crystals of siliconoxide (SiO₂). Since silicon (Si) has a positive polarity and oxide (O₂)has negative polarity, symmetric arraying silicon (Si) and oxide (O₂)leads to establish electric neutrality. However, if a stress is appliedto the silicon oxide (SiO₂) single crystalline piezoelectric material,the electric symmetry is broken and electric charge is generated.

In FIG. 38, the individual axes of a quartz crystal are shown. As shown,X-axes or electric axes are defined by the ridge lines and a Z-axis oroptical axis is defined by an axis being perpendicular to the planeextended by the X-X axes. A single crystalline piezoelectric materialsuch as quartz exhibits specific piezoelectric characteristic and hasspecific polarities with respect to the crystal axes which depend on thearray of molecules of the crystalline piezoelectric material.

The oscillatory type angular rate sensor detects a rotating motion ofthe angular rate sensor by detecting the Coriolis force acting at aright angle with respect to the direction of the oscillations of thetuning folks. Thus, the angular rate sensor is required to havepiezoelectric characteristics for orthogonal two axes and a means forapplying the oscillations and a means for detecting the deformations ata right angle to the applied oscillations due to the Coriolis force.Although a single crystalline tuning folk oscillator is cut from apiezoelectric material is optimized in a view of the polarities on whichthe sensitivity of the sensor depends, it is not easy to obtain the highsensitivity of both directions corresponding to the oscillatorydirection of the tuning folk and the direction of the deformation due tothe Coriolis force. Furthermore, the pair of arms and the root memberbeing components of the tuning folks are susceptible to external shockand external vibrations that occur at frequencies close to the armvibrating frequency. Such disturbances may influence the vibratingstructure and produce erroneous results.

Another known prior art embodiment of an angular rate sensor utilizes asurface acoustic wave (SAW) on a piezoelectric substrate, and moreparticularly to a micro-electro-mechanical (MEM) angular rate sensorthat includes a surface acoustic wave resonator (SAWR) and a surfaceacoustic wave sensors (SAWS).

The basic operating principle of the angular rate sensor utilizing thesurface acoustic waves (SAWs) is as follows. When two progressive SAWswhich propagate on the same axis but in the opposite directions eachother are added, a standing wave is generated on an elastic materialsurface, where the elastic material is composed of the particles such asmolecules. If the standing wave is Rayleigh wave, the wave motion of thesurface particles is distributed such that each particle undergoes aperiodic motion whose orbit is ellipse in the plane which is orthogonalto the surface of the piezoelectric material and parallel to thepropagating direction of the SAWs. Some particles seem to be stationarysince an elliptical orbit is collapsed to a nodal point in the planewhich is orthogonal to the surface of the piezoelectric material andparallel to the propagating direction of the SAWs. At the nodal point,the particles vibrate in the tangential direction. The Coriolis forceacts on the vibrating particles. In order for the angular rate to bedetected, the action of the Coriolis force on the particles has to benear the surface of the material. This occurs due to the distribution ofthe wave motion on the elastic material surface. The secondary wavecaused by the Coriolis force can then be detected and the angular rateis quantified.

If the particle of a mass m, a member composing the elastic material,undergoes a vibrating motion

, an angular rotation

perpendicular to the direction of the vibrating motion

causes a Coriolis force

perpendicular to the directions of both the vibrating motion

and the angular rotation

. Where the vibrating motion

, the angular rotation

, and the Coriolis force

each has three components, in general. Therefore, the effect of theCoriolis force

=2m

×

is a measure of the rate of the angular rotation

. Where a symbol “x” in the above equation represents an externalproduct.

When the angular rate sensor utilizing the surface acoustic waves on asurface of the elastic substrate is rotated, i.e., the elastic substrateis rotated, the Coriolis force is applied to particles vibrating in thestanding wave. The direction of the Coriolis force alternates even ifthe angular rate is constant in time because the velocity of theparticles change temporally depending on the phase of the wave to whichthe particle contributes. The alternating force generates a secondarySAW in the direction orthogonal to the primary standing wave. As theCoriolis force is proportional to both particle vibration velocity andthe angular rate of the substrate, the amplitude of the secondary waveis also proportional to the angular rate. The magnitude of the Coriolisforce can be measured by detecting the amplitude of the secondary SAW.Therefore, the angular rate can be obtained from the magnitude of theCoriolis force. However, if an attempt of detecting the magnitude of theCoriolis force by averaging over a spatial distribution of the vibrationof each particle composing the elastic material is made, it would not beeffective because the Coriolis force acting on each particle cancel oneanother. Therefore, secondary SAWs will never be obtained due to thecancellation.

However, if perturbation masses are arranged on grids at intervals of awavelength such that all masses are arranged at loops (or nodes) of theprimary standing wave, the Coriolis force acting in the area with theperturbation masses are stronger than the Coriolis force acting on thearea without the perturbation masses because the total weight of theparticle and the perturbation masses is heavier than that of particlealone, without the perturbation masses. The coherent alternating forcesgenerated at each perturbation mass build up another SAW whichpropagates in the orthogonal direction to the primary standing wave.

If an elastic material is made of piezoelectric material which convertsthe elastic deformation into an electric field and the secondary wavegenerated by the Coriolis force is detected by two detection electrodes,the output voltage is proportional to the amplitude of the secondarywave. It is preferred that each perturbation mass is deposited with ametal forming an electrode which has a higher mass density.

An elastic surface wave gyroscope and a micro-electro-mechanicalgyroscope both utilizing the above mentioned operating principle aredisclosed in Japanese Unexamined Patent Publication No. 8-334330 toKurosawa and Higuchi and U.S. Pat. No. 6,516,665 to Varadan et al., asshown in FIGS. 39-40. Both the elastic surface wave gyroscope ofKurosawa and Higuchi and the micro-electro-mechanical gyroscope ofVaradan et al. include two pairs of transducers disposed on apiezoelectric substrate with a plurality of metallic dots arranged in anarray, and a pair of reflectors. The plurality of metallic dots servesas a proof mass. One pair of transducers generates the primary surfaceacoustic wave and is called a surface acoustic wave resonator or adriving inter-digital transducer (hereinafter, “driving IDT”). A pair ofreflectors is provided on the outsides of the driving IDT and arearranged to effectively generate the primary surface acoustic wave byreflecting the progressive surface wave generated by the driving IDT onone surface of a piezoelectric substrate. The secondary surface acousticwave generated by the Coriolis force is sensed by another pair oftransducers called a surface acoustic wave sensor or detectinginter-digital transducers (hereafter, “detecting IDTs”). The driving IDTand the detecting IDTs are arranged perpendicularly each other. It ispreferred that the plurality of metallic dots form square electrodeswhich are sandwiched by both the driving IDT and the detecting IDTs.Preferably, another pair of reflectors is provided on the outsides ofthe detecting IDTs and are arranged to effectively generate thesecondary surface acoustic wave.

By using SAWs, it becomes possible for the angular rate sensor to have atwo-dimensional construction, as it can be manufactured only by formingelectrodes deposited on the surface of the piezoelectric materials in amethod of production technology for very-large-scale integrated circuits(VLSIs).

FIG. 39 is a diagram of a prior art embodiment of an angular rate sensordisclosed in Japanese Unexamined Patent Publication No. 8-334330 toKurosawa and Higuchi showing the driving IDT J2, the pairs of reflectorsJ3, J4, J7, and J8, the pair of the detecting IDTs J5, J6, and metallicdots J1, each metallic dot serving as a perturbation mass, formed on thesurface of the piezoelectric substrate of the angular rate sensor. Thedriving IDT J2 and the detecting IDTs J5 are formed such that the teethof the respective comb-shaped electrodes are located in predeterminedpositions corresponding to the loops (nodes) of the elastic surfacewave.

The plurality of metallic dots J1 form almost square electrode on thepiezoelectric substrate. The electrodes sides are parallel to an x-axisand a y-axis which are mutually orthogonal, as shown in FIG. 39. Thedriving IDT J2 which is composed of the comb-shaped electrodes ispositioned on the side of the square electrodes along the x-axis. Theplurality of metallic dots J1 and the driving IST J2 are sandwiched byone of the pairs of reflectors J3, J4. On the surface of thepiezoelectric substrate, a first standing wave of elastic surface wavesare generated within a region J10 of the surface by causing the drivingIDT J2 to generate elastic surface waves propagating in outwarddirections along the x-axis therefrom and by reflecting these elasticsurface waves by the reflectors J3, J4. The plurality of metallic dotsJ1 is disposed on the surface within the region J10. The reflectors J3,J4 are separated from each other by an integral distance equal to onehalf of wavelength of the first standing wave. The driving IDT haselectrodes spaced apart at a distance equal to one half of the firststanding wave.

The detecting IDTs J5, J6 are disposed on the surface of thepiezoelectric substrate, separated from one another by the region J8 anddisposed orthogonally to the pair of the reflectors J3, J4, i.e., thex-axis. In other words, the detecting IDTs J5, J6 are positioned alongthe y-axis. The region J10 and the detecting IDTs J5, J6 are sandwichedby another pair of reflectors J7, J8 along the y-axis. The detectingIDTs J5, J6 are configured to sense a second surface acoustic wave andprovide an output indicative of the characteristics of the secondsurface acoustic wave. In order to intensify the effect of the Coriolisforce on metallic dots J1, each metallic dot is preferably located at anloop, i.e., anti-node of the first standing wave.

FIG. 40 shows a simplified operating principle of the above-mentionedangular rate sensor including the driving IDT J2, the pairs ofreflectors J3, J4, J7, and J8, the pair of the detecting IDTs J5, J6,and metallic dots J1. In FIG. 40, a relationship between an amplitude ofthe first standing wave caused by the driving IDT J2 is shown. Metallicdots J1 have individual metallic dots J11, J12, J13, J14, and J15 asshown in FIG. 40.

If the first surface acoustic wave (SAW) causes metallic dots J1 tooscillate along the x-axis, and the piezoelectric substrate of theangular rate sensor is rotated about the x-axis, the Coriolis force,which is related to the rate of rotation, is detected along the y-axis.The first surface acoustic wave (SAW) on the piezoelectric substrate isgenerated by applying an alternating current (AC) voltage to the drivingIDT J2. The resonant frequency of the first SAW is determined by thedistance between the comb-shaped electrodes which constitute the drivingIDT J2, and ranged, for example, from 10 MHz to several hundreds MHz.The SAW generated by the driving IDT J2 propagates back and forthbetween the reflectors J3, J4 and forms a first standing wave in regionJ10 between the detecting IDTs J5, J6 due to the collective reflectionfrom the reflectors J3, J4. That is, the reflectors J3 and 34 contributeto improve the efficiency of the excitation of the standing wave of theSAW along the x-axis by confining the SAW generated by the driving IDTJ2 in the region J10.

Since each of metallic dots J11, J12, J13, J14, and J15 is designed tobe arranged on a loop, i.e., an anti-node of the first standing wavewithin region J10, particles on which metallic dots J1 are disposed willexperience a larger amplitude of vibration in a z-direction, whichserves as the reference vibrating motion for the angular rate sensor.The z-axis is defined as a direction orthogonal to both the x-axis andthe y-axis.

The metallic dots J1 can be made of a metal film of any metal, such as,for example, gold or aluminum. The metallic dots J1 are subjected tooscillatory motion due to the standing wave excited within the regionJ10 between the reflectors J3, J4. If metallic dots J1 are too large andtoo heavy, they will affect the formation of the standing wave. Hence,although the shape of the individual metallic dot is not important, thesize, position, thickness, and weight are important, and their relativepositions are especially important. When the angular rate sensor isrotated, the Coriolis force acting on the metallic dots J1 produces asecond SAW. Thus, the metallic dots J1 are also spaced such that thephases of the second SAW are coherent and superposed to provide asufficient signal to the detecting IDTs J5, J6.

The metallic dots J1 lie in an x-y plane, in which the x-axis runs fromthe reflector J3 to another reflector J4 and defines a x direction, andthe y-axis runs from the reflector J8 to another reflector J7 anddefines a y direction. In this case, λ₁ is the wavelength of the firstSAW in the x-direction, and λ₂ is the wavelength of the second SAW inthe y-direction. Since the wave length λ₁ in the x-direction and λ₂ inthe y-direction are different to each other due to the wave velocitybeing different in the x and y directions, the spacing between themetallic dots J1 in the x and y directions is also different. Themetallic dots J1 are spaced with a separation of λ₁ in the x-directionand λ₂ in the y-direction.

Furthermore, the metallic dots J1 are interlaced in both the x and ydirections such that the second surface acoustic waves generated by theCoriolis force are coherently superposed. For example, as shown in FIG.40, when the Coriolis force acts on the metallic dots J1, the metallicdots J11, J12, J13, J14 vibrate coherently with the inverse phase of themetallic dot J15 since the metallic dot J15 is located from everymetallic dots J11, J12, J13, J14 by λ₁/2 in the x direction and by λ₂/2in the y direction. So, when the angular rate sensor rotates about thex-axis with rotation rate

_(x), then each of the metallic dots J1 experiences an acceleration of 2

33

_(x) in the y-direction, where

is the velocity vector of the particle on which the individual metallicdot is disposed. The acceleration 2

×

, shows a vector quantity having three components, the first, second,and third component corresponding to the scalar quantity in the x, y,and z directions, respectively.

When the first SAW is excited in the x-direction and the angular ratesensor is rotated about x-axis, the exciting force of the SAW acting onthe metallic dots J11, J12, J13, J14, J15 due to the Coriolis forceleads to excite the second SAW in the y directions. The second SAWgenerated by the Coriolis force acting on the metallic dots J11, J12,J13, J14, J15 is detected by the detecting IDTs J5, J6 since thedetecting IDTs J5, J6 which are spaced with the distance of an integernumber of the wavelength λ₂ and have comb-shaped electrodes. The secondSAW is strengthened being reflected back and forth by the reflectors J7and J8 so that the second standing wave of the second SAW is generatedefficiently. The reflectors J7 and J8 are located such that thedetecting IDTs J5, J6 and the region J10 where the metallic dots J1 aredisposed are sandwiched therebetween. The strength of the secondstanding wave is proportional to the Coriolis force. Therefore thestrength of the second standing wave is proportional to the rotationrate of the angular rate sensor. Then the piezoelectric effect generatesan electric field in the piezoelectric substrate proportional to thestrength of the second standing wave which is detectable by thedetecting IDTs J5, J6 as an electric voltage. Consequently, it ispossible to obtain the rotation rate by measuring the voltage generatedat the detecting IDTs J5, J6.

The elastic surface wave gyroscope of Kurosawa and Higuchi and themicro-electro-mechanical gyroscope of Varadan et al. are realized withtechnology for surface acoustic wave devices, especially, surfaceacoustic wave filters comprising a single crystal piezoelectricsubstrate for propagating a Rayleigh wave. Single crystal piezoelectricmaterials used as the piezoelectric substrate of the elastic surfacewave gyroscope and the micro-electro-mechanical gyroscope includes butis not limited to, lithium niobate (LiNbO₃), lithium tantalite (LiTaO₃),lithium tetraborate (Li₂B₄O₇).

However, even though the above mentioned single crystal piezoelectricmaterials such as lithium niobate (LiNbO₃), lithium tantalite (LiTaO₃),lithium tetraborate (Li₂B₄O₇) are widely used for mechanical filtersutilizing surface acoustic wave, it is not the best way to reduce thesize of the device. Another option is to integrate an angular ratesensor and an external driving circuit thereof into an integrated devicewhen the metallic dots J1 are disposed on one of the above singlecrystal piezoelectric materials for the angular rate sensor. However onedisadvantage of the arrangement comes from the fact that highsensitivity of the angular rate sensor which needs a lot of individualmetallic dots J1, and the relative positions of the metallic dots J1need to be fine tuned so that every metallic dot is positioned at loopsof the secondary SAW caused by the Coriolis force. Furthermore, in orderfor the angular rate sensor to be highly sensitive, a necessary area ofthe region J10 where the metallic dots J1 are disposed must be larger innumber. The mechanical structure of the elastic surface wave gyroscopeof Kurosawa and Higuchi and the micro-electro-mechanical gyroscope ofVaradan et al. is not suitable for reducing the size of the angular ratesensor and integrating an angular rate sensor and a driving circuitthereof into one integrated device.

Further, in a similar structure of the elastic surface wave gyroscope ofKurosawa and Higuchi and the micro-electro-mechanical gyroscope ofVaradan et al., since it is necessary for the plurality of the metallicdots J1 to be located at the standing wave maxima in order to reducetransduction loss, each of the metallic dots J1 will never be too largeor too heavy. However, small and light weight metallic dots J1 can notbring the angular rate sensor into having high sensitivity, because theCoriolis force is proportional to both mass and rotational rate.

In order to improve a downsizing achievement and the scale ofintegration of an angular rate sensor, an attempt has been made tofabricate on the semiconductor substrate the piezoelectric film in whichthe elastic acoustic waves are generated using the same principle asthat of the elastic surface wave gyroscope of Kurosawa and Higuchi andthe micro-electro-mechanical gyroscope of Varadan et al., as shown inFIG. 41. The angular rate sensor of this type comprises a semiconductorsubstrate J20, an insulator film J21, and a piezoelectric film J22. Theinsulator film J21 is deposited on an upper surface of the semiconductorsubstrate J20, and the piezoelectric film J22 is deposited on an uppersurface of the insulator film J21. The metallic dots J23 for sensing theCoriolis force are deposited on an upper surface of the piezoelectricfilms.

However, the angular rate sensor of the type shown in FIG. 41 is noteffective for following reasons. In the structure of the angular ratesensor of this type, an electric polarization in an accumulationdirection in a z-direction or surface electric charges are caused whenthe surface acoustic wave is generated in the piezoelectric film.Wherein, the z-direction is defined as a direction perpendicular to asurface of the piezoelectric film. When the surface acoustic wave isgenerated, particles of the piezoelectric film vibrate in thez-direction due to the piezoelectric effect. Since the piezoelectriceffect converts an inner stress of the piezoelectric film into anelectric field, an amount of the electric polarization or an amount ofelectric charge are proportional to amplitudes of the elastic acousticwaves, then proportional to the stress applied to the piezoelectricfilm. If a piezoelectric constant is positive, positive electric chargeis caused in a region at which a compression stress is generated, andnegative electric charge is caused in other regions at which a tensilestress is applied. As a result of the electric polarization occurred inthe piezoelectric film, a displacement of particles which are located inboth regions at which the compression and tensile stresses are appliedis suppressed. Then, achievement of high sensitivity of the angular ratesensor is difficult in the structure of the angular rate sensor of thetype which is organized in such a way that the piezoelectric film J22and the metallic dots J23 are accumulated on the semiconductor substrateJ20.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation in the prior art, and has as its object to provide an angularrate sensor comprising a semiconductor substrate and a piezoelectricfilm accumulated on the semiconductor substrate and having features suchas compactness, high sensitivity, high reliability, high stability, highprecision, and high mass-production capability by suppressing anelectric polarization in the perpendicular direction to thepiezoelectric film.

In order to achieve the above object, according to the first aspect ofthe present invention, there is provided an angular rate sensorcomprising a semiconductor substrate, a first electrode formed on anupper surface of the semiconductor substrate, a piezoelectric filmformed on the whole region of the first electrode, metallic dots servingas perturbation masses disposed on the opposite surface of thepiezoelectric film to a surface contacting with the semiconductorsubstrate, each metallic dot being capable of vibrating in theperpendicular direction to the piezoelectric film, a driving IDT forcausing an elastic acoustic wave in the piezoelectric film in anresponse to externally applied driving voltage disposed on thepiezoelectric film, a pair of reflectors sandwiching the driving IDT andthe metallic dots reflecting the elastic acoustic wave caused by thedriving IDT to form a first standing wave of the elastic acoustic waveaccompanying the vibration of the metallic dots resonating with anamplitude of the elastic acoustic wave, and detecting IDTs for detectinga second standing wave generated by a Coriolis force acting on themetallic dots which vibrate, resonating with the amplitude of theelastic acoustic wave.

In the angular rate sensor having the first electrode sandwiched betweenthe semiconductor substrate and the piezoelectric film, the polarizationin the z-direction, which is defined as a perpendicular direction to thesurface of the piezoelectric film, can be suppressed even if an innerstress in the piezoelectric film is generated by the Coriolis forceacting on vibrating particles in the piezoelectric substrate and thenthe inner stress generates an electric field in the z-direction which isan origin of a surface electric charge, i.e., the electric polarizationof the piezoelectric film in the z-direction due to neutralizationeffect by the first electrode. In more detail, if surface electriccharge is generated at the lower surface of the piezoelectric filmconnecting to the first electrode, surface electric charge escapes fromthe piezoelectric film and is thus neutralized via the first electrode.Therefore, a limitation of an amount of a displacement of the vibratingparticle in the piezoelectric substrate, especially of the metallic dotsvibrating in the z-direction due to the polarization in the z-directionof the piezoelectric film is eliminated. In consequence, it is possibleto realize an angular rate sensor having high sensitivity.

Further, a basic structure of the angular rate sensor according to thepresent invention on which the driving IDT, reflectors, and thedetecting IDTs take, resembles that taken by the prior arts. Hence, itbecomes possible to reduce a size of the angular rate sensor because anecessary size for obtaining a sufficient sensitivity becomes smallerdue to higher sensitivity of the angular rate sensor according to thepresent invention. Therefore the angular rate sensor having highsensitivity can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

In order to achieve the above object, according to the second aspect ofthe present invention, there is provided an angular rate sensor furthercomprising a second electrode sandwiched between the piezoelectric filmand the plurality of metallic dots and covering a whole region where themetallic dots are formed.

In the angular rate sensor having the second electrode sandwichedbetween the piezoelectric film and the plurality of metallic dots, thepolarization in the z-direction can be suppressed due to neutralizationeffect by the first electrode even if an inner stress in thepiezoelectric film is generated by the Coriolis force acting onvibrating particles in the piezoelectric substrate and then the innerstress generates an electric field in the z-direction which causes asurface electric charge at an upper and a lower surface of thepiezoelectric film or an electric polarization of the piezoelectric filmin the z-direction. In more detail, if surface electric charge isgenerated at the upper surface of the piezoelectric film connecting tothe second electrode, surface electric charge escapes from thepiezoelectric film and thus is neutralized via the second electrode.Therefore, a limitation of an amount of a displacement of the vibratingparticle in the piezoelectric substrate, especially of the vibratingmetallic dots in the z-direction due to the polarization in thez-direction of the piezoelectric film, is eliminated. In consequence, itis possible to realize an angular rate sensor having high sensitivity.Further the angular rate sensor having high sensitivity can beminiaturized with production technology for very-large-scale integratedcircuits (VLSIs).

In order to achieve the above object, according to the third aspect ofthe present invention, there is provided an angular rate sensor furthercomprising a contact hole formed in the piezoelectric film forelectrically connecting the first electrode sandwiched between thesemiconductor substrate and the piezoelectric film to the secondelectrode formed on the upper surface of the piezoelectric film butbelow the metallic dots so as to keep the same electric potential of thefirst electrode with that of the second electrode.

The angular rate sensor in which the contact holes are formed in thepiezoelectric film such a way that the electric potential of the firstand second electrodes connected to the lower and upper surface of thepiezoelectric film respectively maintains at the same level. This cansuppress the generation of a difference of the electric potentialbetween the upper and lower surface of the piezoelectric film. Hence, alimitation of the displacement of the vibrating particle in thepiezoelectric substrate, especially of the vibrating metallic dots inthe z-direction due to the polarization in the z-direction of thepiezoelectric film, is eliminated. In consequence, it is possible torealize an angular rate sensor having high sensitivity.

In the angular rate sensor of this type, it is preferable that eachcontact hole is connecting to the corresponding metallic dot. In otherwords, the individual metallic dot is formed on the top of the contacthole. If the contact holes are formed at the position where the metallicdots are disposed, some portion of a weight of the contact holescontributes to a perturbation mass without affecting the characteristicsof the elastic acoustic wave. Therefore, it is possible to realize anangular rate sensor having high sensitivity.

In order to achieve the above object, according to the forth aspect ofthe present invention, there is provided an angular rate sensor havingopenings for accommodating a perturbation weight in the piezoelectricfilm.

In the angular rate sensor having the openings for accommodating theperturbation weight in the piezoelectric film disposed on thesemiconductor substrate, the piezoelectric material is expelled from theopenings. Therefore, even if the standing wave of the elastic acousticwave generates surface electric charges at the surfaces of thepiezoelectric film by the piezoelectric effect at a region where themetallic dots and the openings are positioned, the surface electriccharges will never affect the vibration of the perturbation weight inthe z-direction. As a result, a limitation of the displacement of thevibrating perturbation weights accommodated into the openings in thez-direction is eliminated. In consequence, it is possible to realize anangular rate sensor having high sensitivity.

In the modification of the angular rate sensor of this type, it ispreferable that the angular rate sensor having the openings foraccommodating the perturbation weight in the piezoelectric film furthercomprises the first electrode sandwiched between the semiconductorsubstrate and the piezoelectric film.

In all aspects of the present invention mentioned above, it ispreferable that the perturbation masses or the perturbation weight aremade of metal or metallic alloy whose mass density is greater than 13.5g/cm³. The metals or metallic alloys being suitable ones for theperturbation masses or the perturbation weight include platinum (Pt),tungsten (W), and gold (Au). In such a configuration of the perturbationmasses or the perturbation weight, the Coriolis force acting on theperturbation masses or the perturbation weight is emphasized so as toincrease the displacement of the perturbation masses or the perturbationweight in the z-direction. In consequence, it is possible to realize anangular rate sensor having high sensitivity. Further the angular ratesensor having high sensitivity can be miniaturized with productiontechnology for very-large-scale integrated circuits (VLSIs).

In order to achieve the above object, according to the fifth aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor comprising steps of a preparing step for preparingthe semiconductor substrate, a first electrode forming step fordepositing the first electrode on the semiconductor substrate at leastover a region above which a perturbation weight is formed, apiezoelectric film forming step for depositing the piezoelectric film onat least one of the first electrode and the semiconductor substrate, anda fabricating step for fabricating a plurality of features on thepiezoelectric film, wherein the plurality of features includes metallicdots serving as the perturbation masses, a driving IDT for causing aelastic acoustic wave in the piezoelectric film, reflectors forreflecting the elastic acoustic wave so as to form a first and a secondstanding wave, and detecting IDTs for detecting a second standing wavecaused by the Coriolis force acting on the first standing wave. Inconsequence, it is possible to realize an angular rate sensor which canbe miniaturized with production technology for very-large-scaleintegrated circuits (VLSIs).

In a modification of the method for manufacturing an angular ratesensor, the fabricating step for fabricating a plurality of features onthe piezoelectric film further comprises a first fabricating step forfabricating the driving IDT, reflectors, and the detecting IDTs, and asecond fabricating step for fabricating the perturbation masses.

In order to achieve the above object, according to the sixth aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor further comprising a step of a second electrodeforming step for depositing the second electrode sandwiched between thepiezoelectric film and the metallic dots at least over a region whichcovers a region where the metallic dots are formed. In consequence, itis possible to realize an angular rate sensor which can be miniaturizedwith production technology for very-large-scale integrated circuits(VLSIs).

In order to achieve the above object, according to the seventh aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor having the contact hole in the piezoelectric filmfurther comprising a step of a forming the contact hole in thepiezoelectric film wherein the second electrode forming step isconfigured to electrically connect the second electrode to the firstelectrode which is sandwiched between the semiconductor substrate andthe piezoelectric film via the contact hole. If the contact hole isarranged such that the metallic dot is in alignment with the contacthole in the z-direction, a weight of a portion of the second electrodewhich is located just below one of the metallic dots contributes to theperturbation masses. Therefore, it is possible to realize an angularrate sensor which can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

In order to achieve the above object, according to the eighth aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor comprising steps of a preparing step for preparingthe semiconductor substrate, a piezoelectric film forming step fordepositing the piezoelectric film on the semiconductor substrate, asecond electrode forming step for depositing the second electrode on thepiezoelectric film at least over a region above which a perturbationweight is formed, a fabricating step for fabricating a plurality offeatures on the piezoelectric film, wherein the plurality of featuresincludes a driving IDT for causing a elastic acoustic wave in thepiezoelectric film, reflectors for reflecting the elastic acoustic waveso as to form a first and a second standing wave, and detecting IDTs fordetecting a second standing wave caused by the Coriolis force acting onthe first standing wave, and a further fabricating step for fabricatingthe metallic dots serving as the perturbation weight on the secondelectrode.

In a modification of the above methods for manufacturing angular ratesensors, it is preferable that methods further comprise a step of aninsulator film forming step for forming the insulator film on thesurface of the piezoelectric film. Thus, it is possible to realize anangular rate sensor which can be miniaturized with production technologyfor very-large-scale integrated circuits (VLSIs).

In order to achieve the above object, according to the ninth aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor comprising steps of a preparing step for preparingthe semiconductor substrate, a piezoelectric film forming step fordepositing the piezoelectric film on the semiconductor substrate, anopening forming step for forming the opening in the piezoelectric filmover a region above which a perturbation masses are disposed, afabricating step for fabricating a plurality of features on thepiezoelectric film, wherein the plurality of features includes a drivingIDT for causing a elastic acoustic wave in the piezoelectric film,reflectors for reflecting the elastic acoustic wave so as to form afirst and a second standing wave, and detecting IDTs for detecting asecond standing wave caused by the Coriolis force acting on the firststanding wave, and a further fabricating step for fabricating themetallic dots serving as the perturbation masses on a surface of theopening. Hence, it is possible to realize an angular rate sensor whichcan be miniaturized with production technology for very-large-scaleintegrated circuits (VLSIs).

In a first modification of this method for manufacturing angular ratesensors, it is preferable that the method further comprise a step of afirst insulator film forming step for forming the first insulator filmon the surface of the piezoelectric film. In a second modification ofthis method for manufacturing angular rate sensors, it is preferablethat the method further comprises a first electrode forming step forforming the first electrode either on an upper surface of thesemiconductor substrate or on an upper surface of the first insulatorfilm wherein the piezoelectric film forming step is configured to formthe piezoelectric film on the surface of the first electrode. In a thirdmodification of this method for manufacturing angular rate sensors, itis preferable that the method further comprising a second insulator filmforming step for forming the second insulator film either on an uppersurface of the piezoelectric film exposed due to the existence of theopening in the piezoelectric film or on am upper surface of thepiezoelectric film. In consequence, it is possible to realize an angularrate sensor which can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

In order to achieve the above object, according to the tenth aspect ofthe present invention, there is provided an angular rate sensorcomprising a semiconductor substrate, a first electrode formed on anupper surface of the semiconductor substrate, a piezoelectric filmformed on the whole region of the first electrode, metallic dots servingas perturbation masses disposed on the opposite surface of thepiezoelectric film to a surface contacting with the semiconductorsubstrate, each metallic dot being capable of vibrating in theperpendicular direction to the piezoelectric film, a driving IDT forcausing an elastic acoustic wave in the piezoelectric film in responseto an externally applied driving voltage disposed on the piezoelectricfilm, a pair of reflectors sandwiching the driving IDT and the metallicdots reflecting the elastic acoustic wave caused by the driving IDT toform a first standing wave of the elastic acoustic wave withaccompanying the vibration of the metallic dots resonating with anamplitude of the elastic acoustic wave, and detecting IDTs for detectinga second standing wave generated by a Coriolis force acting on themetallic dots which vibrate resonating with the amplitude of the elasticacoustic wave, wherein a mass density in a region where the metallicdots are disposed is larger than that in a further region where thedriving IDT, reflectors, and detecting IDTs are disposed. Thisconfiguration leads to an effect where the Coriolis force acting on themetallic dots is increased since a magnitude of the Coriolis force isproportional to the mass density. Then both amplitude of an elasticacoustic wave generated by the Coriolis force and the velocity ofparticles located just below the metallic dots are also increased as themagnitude of the Coriolis force increases. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.

In a modification of the angular rate sensor of this type, there isprovided an angular rate sensor having the first electrode sandwichedbetween the semiconductor substrate and the piezoelectric film. In theangular rate sensor having the first electrode, an electric polarizationin the z-direction can be suppressed due to neutralization effect by thefirst electrode even if an inner stress in the piezoelectric film isgenerated by the Coriolis force acting on vibrating particles in thepiezoelectric substrate and then the inner stress generates an electricfield in the z-direction which is a origin of a surface electric chargeat an upper and a lower surface of the piezoelectric film or an electricpolarization of the piezoelectric film in the z-direction. In moredetail, if the surface electric charge is generated at the lower surfaceof the piezoelectric film being connected to the first electrode, thesurface electric charge escapes from the piezoelectric film and is thusneutralized via the first electrode. Therefore, a limitation of anamount of a displacement of the vibrating particle in the piezoelectricsubstrate, especially of the metallic dots vibrating in the z-directiondue to the polarization in the z-direction of the piezoelectric film iseliminated. In consequence, it is possible to realize an angular ratesensor having high sensitivity because a mass density in a region wherethe metallic dots are disposed is larger than that in a further regionwhere the driving IDT, reflectors, and detecting IDTs are disposed. Thiscan lead to the realization of highly sensitive angular rate sensor.

In a further modification of the angular rate sensor of this type, thereis provided an angular rate sensor having a second electrode sandwichedbetween the piezoelectric film and the plurality of metallic dots. Thiscan also make it possible to realize an angular rate sensor having highsensitivity. It is also preferable that the angular rate sensor having aplurality of trenches in the semiconductor substrate into which theperturbation masses are accommodated. In this configuration, theperturbation masses are located under the piezoelectric film.

In order to achieve the above object, according to the eleventh aspectof the present invention, there is provided an angular rate sensorcomprising a plurality of trenches whose exposed surface is covered byan insulator film in the semiconductor substrate, a plurality offeatures including include the driving IDT, reflectors, and detectingIDTs and being fabricated in the trenches, and a piezoelectric filmarranged so as to cover the plurality of features such as the drivingIDT, reflectors, and detecting IDTs, wherein a mass density in a regionwhere the metallic dots are disposed is larger than that in a furtherregion where the driving IDT, reflectors, and detecting IDTs aredisposed. This configuration leads to an effect where the Coriolis forceacting on the metallic dots is increased since a magnitude of theCoriolis force is proportional to the mass density. Then both amplitudeof an elastic acoustic wave generated by the Coriolis force andvelocities of particles located just below the metallic dots are alsoincreased as the magnitude of the Coriolis force increases. Inconsequence, it is possible to realize an angular rate sensor havinghigh sensitivity.

As is in the angular rate sensor comprising the plurality of featuresincluding the driving IDT, reflectors, and detecting IDTs, and beingdisposed on the piezoelectric film, the same advantages are obtained inan angular rate sensor having the plurality of feature disposed belowthe piezoelectric film wherein a mass density in a region where themetallic dots are disposed is larger than that in a further region wherethe plurality of feature including the driving IDT, reflectors, anddetecting IDTs are disposed.

This configuration in which the plurality of feature including thedriving IDT, reflectors, and detecting IDTs are disposed below thepiezoelectric film can be realized through the formation of formingtrenches, accommodating the perturbation masses thereinto, and forming apiezoelectric film so as to cover the trenches.

Further, as a modification of the angular rate sensor according to theeleventh aspect of the present invention, a conductive film is formed onthe piezoelectric film. It becomes possible for the angular rate sensorhaving the conductive film formed on the piezoelectric film to generateand keep a predetermined voltage between the driving IDT and theconductive film such that electric power is efficiently inputted intothe piezoelectric film since the electric potential level of thepiezoelectric film is able to be kept constant. Further, if theconductive film is connected to a ground, the conductive film acts as ashield. This fact leads to an expectation that the angular rate sensoris insensitive to external electric noise. Therefore, it becomespossible that the angular rate sensor to have high sensitivity since theelectric signal level is increased due to an electronic noise reductioneffect of the conduction film.

In a modification of the angular rate sensor including the conductivefilm, the angular rate sensor is further comprised of the openings overwhich the conductive film is expelled for accommodating the perturbationweight.

In the tenth and eleventh aspects of the present invention mentionedabove, a mass density in a region where the perturbation masses aredisposed is larger than that in a further region where the driving IDT,reflectors, and detecting IDTs are disposed. This configuration leads toan effect where the Coriolis force acting on the metallic dots isincreased since a magnitude of the Coriolis force is proportional to themass density. Then both amplitude of an elastic acoustic wave generatedby the Coriolis force and velocities of particles located just below themetallic dots are also increased as the magnitude of the Coriolis forceincreases. In consequence, it is possible to realize an angular ratesensor having high sensitivity. In order to achieve the just mentionedcondition, there is a method in which the perturbation masses or theperturbation weight are made of metal or metallic alloy whose massdensity is greater than 13.5 g/cm³. The metals or metallic alloys beingsuitable ones for the perturbation masses or the perturbation weightinclude platinum (Pt), tungsten (W), and gold (Au). In such aconfiguration of the perturbation masses or the perturbation weight, theCoriolis force acting on the perturbation masses or the perturbationweight is emphasized so as to increase the displacement of theperturbation masses or the perturbation weight in the z-direction. Inconsequence, it is possible to realize an angular rate sensor havinghigh sensitivity. Further the angular rate sensor having highsensitivity can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

There is a further method for achieving the condition that a massdensity in a region where the perturbation masses are disposed is largerthan that in a further region where the driving IDT, reflectors, anddetecting IDTs, thickness of the perturbation masses are larger thanthat in the further region. If the perturbation masses, the driving IDT,reflectors, and detecting IDTs are made of the same metal or metallicalloy, the above condition can be satisfied.

In order to achieve the above object, according to the twelfth aspect ofthe present invention, there is provided a method for manufacturing anangular rate sensor comprising steps of a preparing step for preparingthe semiconductor substrate, a first electrode forming step fordepositing the first electrode on the semiconductor substrate at leastover a region above which a perturbation weight is formed, apiezoelectric film forming step for depositing the piezoelectric film onthe first electrode or the semiconductor substrate, a further preparingstep for preparing a first and a second materials, a mass density of thesecond material is larger than that of the first material, a fabricatingstep for fabricating a plurality of features made of the first materialon the piezoelectric film, wherein the plurality of features includes adriving IDT for causing a elastic acoustic wave in the piezoelectricfilm, reflectors for reflecting the elastic acoustic wave so as to forma first and a second standing wave, and detecting IDTs for detecting asecond standing wave caused by the Coriolis force acting on the firststanding wave, and a further fabricating step for fabricating theperturbation masses made of the second material serving as theperturbation masses on a surface of the piezoelectric film. Therefore, amass density in a region where the perturbation masses are disposed islarger than that in a further region where the plurality of featureincluding the driving IDT, reflectors, and detecting IDTs are disposed.Hence, it is possible to realize an angular rate sensor which can beminiaturized with production technology for very-large-scale integratedcircuits (VLSIs).

In an angular rate sensor manufactured by a method according to thetwelfth aspects of the present invention mentioned above, the Coriolisforce acting on the perturbation masses is increased since a magnitudeof the Coriolis force is proportional to the mass density. Then bothamplitude of an elastic acoustic wave generated by the Coriolis forceand velocities of particles located just below the metallic dots arealso increased as the magnitude of the Coriolis force increases. Inconsequence, it is possible to realize an angular rate sensor havinghigh sensitivity.

In a modification of the method for manufacturing the angular ratesensor according to the twelfth aspects of the present invention, themethod further comprises a step of a first electrode forming step forforming the first electrode on the semiconductor substrate and below thepiezoelectric film over a region above which the perturbation masses isdisposed.

In the angular rate sensor having the first electrode, an electricpolarization in the z-direction can be suppressed due to neutralizationeffect by the first electrode. In more detail, if surface electriccharge is generated at the lower surface of the piezoelectric filmconnecting to the first electrode, surface electric charge escapes fromthe piezoelectric film and thus is neutralized via the first electrode.Therefore, a limitation of an amount of a displacement of the vibratingparticle in the piezoelectric substrate, especially of the metallic dotsvibrating in the z-direction due to the polarization in the z-directionof the piezoelectric film is eliminated. In consequence, it becomespossible to realize an angular rate sensor having high sensitivity.

In the method for manufacturing the angular rate sensor according to thetwelfth aspects of the present invention, it is preferable that thefirst electrode forming step is configured to make the first electrodeof one of impurity doped polysilicon, aluminum (Al), aluminum-basealloy, titanium (Ti), titanium-base alloy, tungsten, tungsten-basealloy, molybdenum, and molybdenum-base alloy.

In a modification of the method for manufacturing the angular ratesensor according to the twelfth aspects of the present invention, thefirst electrode forming step is configured to form the first electrodeon the piezoelectric film and below the perturbation masses. In thiscase the first electrode covers a region over which the perturbationmasses are disposed.

In a further modification of the method for manufacturing the angularrate sensor according to the twelfth aspects of the present invention,there is provided a method for manufacturing an angular rate sensorfurther comprising steps of a trench forming step for forming aplurality of trenches in the semiconductor substrate, a insulator filmforming step for disposing the insulator film on an exposed surface ofevery trench, a perturbation mass forming step for forming theperturbation masses in the trenches whose surfaces are coated by theinsulator film, and a piezoelectric film forming step for forming thepiezoelectric film so as to cover the plurality of the trenches, theperturbation masses and the semiconductor substrate. In thisarrangement, the perturbation masses are located under the piezoelectricsubstrate.

In order to achieve the above object, according to the fourteenth aspectof the present invention, there is provided a method for manufacturingan angular rate sensor comprising steps of a preparing step forpreparing the semiconductor substrate, a trench forming step for forminga plurality of trenches in the semiconductor substrate, a insulator filmforming step for disposing the insulator film on an exposed surface ofevery trench, a further preparing step for preparing a first and asecond materials, a mass density of the second material is larger thanthat of the first material, a fabricating step for fabricating aplurality of features made of the first material in some trenches,wherein the plurality of features includes a driving IDT for causing aelastic acoustic wave in the piezoelectric film, reflectors forreflecting the elastic acoustic wave so as to form a first and a secondstanding wave, and detecting IDTs for detecting a second standing wavecaused by the Coriolis force acting on the first standing wave, and afurther fabricating step for fabricating the perturbation masses made ofthe second material serving as the perturbation masses in othertrenches, and a piezoelectric film forming step for forming thepiezoelectric film so as to cover the plurality of the trenches, theperturbation masses and the semiconductor substrate. Therefore, a massdensity in a region where the perturbation masses are disposed is largerthan that in a further region where the plurality of feature includingthe driving IDT, reflectors, and detecting IDTs are disposed. Hence, itis possible to realize an angular rate sensor which can be miniaturizedwith production technology for very-large-scale integrated circuits(VLSIs).

In a modification of a method according to the fourteenth aspect, afabricating step for fabricating a plurality of features made of thefirst material in some trenches, wherein the plurality of featuresincludes a driving IDT, the reflectors, and the detecting IDTs, furthercomprises an insulator film forming step for forming the insulator filmon exposed surfaces of the trenches formed in the semiconductorsubstrate, wherein the further fabricating step is configured tofabricate the perturbation masses in the trenches whose exposed surfacesare coated by the insulator film so as to cover the plurality of thetrenches, the perturbation masses and the semiconductor substrate.

In a further modification of a method according to the fourteenthaspect, the method for manufacturing the angular rate sensor furthercomprises a conductive film on the piezoelectric substrate. It ispossible for the angular rate sensor having the conductive film formedon the piezoelectric film to generate and keep a predetermined voltagebetween the driving IDT and the conductive film such that electric poweris efficiently inputted into the piezoelectric film since the electricpotential level of the piezoelectric film is able to be kept constant.Further, if the conductive film is connected to a ground, the conductivefilm acts as a shield. This fact leads to an expectation that theangular rate sensor is insensitive to external electric noise.Therefore, it is now possible that the angular rate sensor has highsensitivity since the electric signal level is increased due to anelectronic noise reduction effect of the conduction film.

In a modification of the angular rate sensor including the conductivefilm, the angular rate sensor is further comprised of the openings overwhich the conductive film is expelled for accommodating the perturbationweight.

In order to achieve the above object, according to the fifteenth aspectof the present invention, there is provided an angular rate sensorcomprising a piezoelectric substrate having an upper and a lowersurfaces and vibrating such as an elastic wave in response to anelectric signal due to a piezoelectric effect by which an electricenergy is converted into a mechanical deformation energy of thepiezoelectric substrate and vice versa, a first electrode formed on thelower surface of the piezoelectric substrate, a second electrode formedin the upper surface of the piezoelectric substrate such that the secondelectrode is opposed to the first electrode via the piezoelectricsubstrate and configured to serve as a perturbation mass on which aCoriolis force acts if the perturbation mass has a velocity thereofrelative to the piezoelectric substrate and the piezoelectric substrateis rotated, and detectors for detecting electric signals generated bythe Coriolis force and then related to physical quantities aboutrotation phenomena of the piezoelectric substrate. In the angular ratesensor having the second electrode configured to serve as theperturbation mass, the second electrode vibrate in a z-direction definedas a perpendicular direction to the upper surface of the piezoelectricsubstrate due to the piezoelectric effect if alternative current (AC)voltage is applied between the first and second electrodes. Thus, itbecomes possible that information of rotation of the piezoelectricsubstrate is obtained via the vibration of the second electrode formedon the upper surface of the piezoelectric substrate. One of advantagesof the angular rate sensor of the above described type is that anecessary area for a perturbation mass becomes to be smaller than thatfor metallic dots serving as perturbation masses on which the Coriolisforce acts when an elastic acoustic wave is excited along a paralleldirection to the upper surface of the piezoelectric substrate sinceelastic acoustic wave is generated in the z-direction, i.e., in theperpendicular direction to the upper surface of the piezoelectricsubstrate. Another advantage of the angular rate sensor of the abovedescribed type is that a driving IDT and reflectors are not necessary.Only detecting IDTs are necessary. The driving IDT and reflectors do notnecessary to be arranged such that all of the driving IDT, reflectors,and a region wherein metallic dots serving as perturbation masses on astraight line. Therefore, downsizing of angular rate sensors andintegrating an angular rate sensor and an external driving circuitthereof into a small size integrated device are simultaneously achieved.

In a modification of the angular rate sensor of the above mentioned typehaving the second electrode configured to serve as the perturbationmass, the detecting IDTs further comprises a first detecting IDTs and asecond detecting IDTs, each of the first and the second IDTs arecomposed of the plurality of the electrodes. The first detecting IDTsand the second detecting IDTs are located such that the region whereinthe second electrode serving as the perturbation mass is sandwichedbetween the first and second IDTs.

In the angular rate sensor having the above mentioned arrangement,rotation rate is measured based on the difference between a measuredelectric power detected by the first detecting IDT and that of thesecond detecting IDT so that effects of external noise in elasticacoustic waves are removed from final results of the measurement.

In a further modification of the angular rate sensor of the abovementioned type having the second electrode configured to serve as theperturbation mass, the detecting IDTs further comprises a thirddetecting IDT and a forth detecting IDT. Each of the third and the forthIDTs are composed of a plurality of electrodes. The third and the forthIDT is located such that the second electrode serving as theperturbation mass is sandwiched therebetween and the first and thesecond IDTs are orthogonally arranged on the upper surface of thepiezoelectric substrate.

This configuration of the first, second, third, and forth detecting IDTsleads to the possibility of detecting rotation rate about multipleorthogonal axes. Thus, this configuration is capable of reducing effectsof direct elastic acoustic waves generated by applying external ACvoltages on measuring voltage relating to elastic acoustic wavesgenerated by the Coriolis force.

Further, the above configuration of the first, second, third, and forthdetecting IDTs is capable of reducing an unnecessary contribution fromthe direct elastic acoustic waves. The first and second detecting IDTsprovide a first output including information about a difference inoutput voltages detected by the first and second IDTs. Similarly, thethird and forth IDTs provide a second output including information abouta difference in output voltages detected by the third and forth IDTs. Ifangular rate is obtained using the first and second outputs, the angularrate sensor becomes insensitive to the direct elastic acoustic waveswhich are not experienced of the scattering by the Coriolis force.

In a further modification of the angular rate sensor of the abovementioned type having the second electrode configured to serve as theperturbation mass, the piezoelectric substrate is replaced with a thinpiezoelectric film having upper and lower surfaces. In this case, theangular rate sensor further comprises a supporting member whose uppersurface is covered by the first electrode. The lower surface of the thinpiezoelectric film covers the upper surface of the first electrode.

It is preferable that the supporting member is made of an insulatingsubstrate or a semiconductor substrate having a high resistivity.

It is further preferable that a via hole is formed from thesemiconductor substrate so as to obtain a structure in which the lowersurface of the first electrode is exposed to the air. In thisconfiguration of the angular rate sensor, the second electrode servingas the perturbation mass is capable of vibrating in the z-direction withlarge amplitude so as to obtain high sensitivity to the Coriolis forceacting on the perturbation mass, i.e., to rotation rate.

In this configuration, an unnecessary vibration of the perturbation massis generated if the via hole is too large. Thus, it is preferable that alength of the via hole along a x-direction, which is defined by adirection in which the first and second detecting IDTs are in alignmentwith each other, is shorter than a spacing between the first and secondIDTs and the ends of the perturbation mass lie off the via hole alongthe x-direction, so that the unnecessary vibration of the perturbationmass is suppressed.

In a situation where direction is not in the x-direction, a longerlength of the via hole overlapping edges of the first and second IDTsalong the y-direction, which is defined by a direction orthogonal to thex-direction on the surface of the thin piezoelectric film, ispreferable. In such a configuration of the via hole, it is possible forthe first and second IDTs to output a higher lever of electric signalsproportional to the Coriolis force acting on the perturbation mass whenthe thin piezoelectric film rotates about the x-axis. Thus, it ispreferable that the ends of the via hole lie off the perturbation massalong the y-direction, so that the unnecessary vibration of theperturbation mass is suppressed.

If the angular rate sensor is arranged to have the first, second, third,and forth detecting IDTs so as to detect rotation rate about multipleorthogonal axes, the ends of the perturbation mass both along thex-direction and the y-direction preferably lie off the via hole. In thisarrangement, the unnecessary vibration of the perturbation mass issuppressed.

The second electrode also serving as the perturbation mass preferablyconsists of a single electrode so that a mass density of a region wherethe second electrode is formed is increased. Therefore, still preferablythe second electrode is formed in a rectangular shape in order toincrease the mass density of the region where the second electrode isformed. The fact that the mass density of the region where the secondelectrode is formed is large brings the second electrode to vibrate withlarge amplitude in a z-direction defined as a piezoelectric filmthickness direction. When the thin piezoelectric film rotates, electriccurrent relating to the Coriolis force acting on the second electrodevibrating along the z-direction whose amplitude is proportional tovibrating velocity thereof is generated. Therefore, the larger theamplitude of the vibrating velocity of the second electrode is, thehigher the sensitivity of the angular rate sensor is, since the secondelectrode vibrates in the z-direction with a larger amplitude.

In a modification of the angular rate sensor of this type, a firstelectrode is formed on the lower surface of the thin piezoelectric filmin a region above which the second electrode is disposed.

In this configuration, the first electrode is only formed on the otherregion above which the detecting IDTs are disposed. Therefore, a regionbelow the detecting IDTs in the thin piezoelectric film can avoidelectric fields and elastic acoustic waves because the first electrodeis not there below the detecting IDTs.

In a further modification of the angular rate sensor having the secondelectrode also serving as the perturbation mass, a thin insulator filmis formed so as to cover the upper surface of the thin piezoelectricfilm on which the second electrode is disposed.

Preferably, the thin insulator film is disposed on the upper surface ofthe thin piezoelectric film over a region on which the second electrodealso serving as the perturbation mass is disposed. This arrangement ofthe thin insulator film enables a reduction of electric current leakagebetween the first and second electrodes. There is a further advantage ofthe angular rate sensor of this type where weight of a part of the thininsulator film located below the second electrode contributes to theperturbation mass in addition to the weight of the second electrode sothat a high sensitivity of the angular rate sensor is achieved.

Still further, the angular rate sensor according to the presentinvention, the piezoelectric substrate and the thin piezoelectric filmis made of one of aluminum nitride (AlN), zinc oxide (ZnO), zirconatetitante (PZT), lead titanate (PT), lithium tantalite (LiTaO₃), andlithium tantalite (LT). If the thin piezoelectric film is made of AlN,an integration of the other functional device, such as complementarymetal-oxide-semiconductors (CMOS), into the angular rate sensor ispossible to achieve without taking account of an environmental metalpollution.

Still further, the angular rate sensor according to the presentinvention, if at least one of the first electrode or the secondelectrode also serving as the perturbation mass is made of one ofaluminum (Al), aluminum (Al)-silicon (Si) alloy, aluminum (Al)-silicon(Si)-copper (Cu) alloy, and impurity-doped poly-silicon, it is possibleto be form the first electrode with a semiconductor productiontechnology with contributing prevention of environmental metalpollution.

Still further, in the angular rate sensor according to the presentinvention, at least one of the first electrode and the second electrodealso serving as the perturbation mass is made of one of aluminum (Al),platinum (Pt), tungsten (W), and rubidium (Ru), mass density of thefirst and second electrodes is increased so that total weight of thefirst and second electrodes is grown.

Still further, in the angular rate sensor according to the presentinvention, it is preferable that the second electrode is composed of aplurality of metallic island films which are connected electrically toeach other and are driven simultaneously by an external electric supply.

In this arrangement of the angular rate sensor, elastic acoustic wavesgenerated at individual electrodes composing the second electrode by theCoriolis force are synchronously emphasized. Therefore, high sensitivityof the angular rate sensor is achieved.

Still further, an angular rate sensing device is provided by integratinga plurality of an angular rate sensors into a single device such that afinal result of measured angular rate is obtained based on electricsignals outputted from the plurality of the angular rate sensors.Therefore, an angular rate sensing device producing an accurate measuredresult and having high sensitivity is provided.

Still further, an angular rate sensor according to the present inventionutilizes elastic acoustic wave generated in elastic materials includingpiezoelectric film. In the case where the elastic acoustic wave iscaused in the piezoelectric film, it is possible to neglect a cuttingand propagating direction of a piezoelectric material duringmanufacturing the angular rate sensor. This fact leads to realize thehigh sensitivity and high productivity of the angular rate sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the invention, itsconstruction and operation will be described in detail based on theaccompanying drawings, in which:

FIG. 1A and FIG. 1B show a structure of an angular rate sensor accordingto a first embodiment of the present invention;

FIG. 2 shows a Coriolis force acting on each of the perturbation massesproduces, if the angular rate sensor according to the first embodimentof the present invention undergoes a rotary motion while an elasticacoustic waves of the piezoelectric film is generated by the drivingelectrodes;

FIG. 3 shows a cross-sectional view of the angular rate sensor accordingto the first embodiment taken along line A-A in FIG. 1A;

FIG. 4 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 1A and FIG. 1B according to the first embodiment ofthe present invention;

FIGS. 5A to 5C illustrate the steps performed in the method formanufacturing the angular rate sensor according to the first embodiment;

FIG. 6 shows a cross-sectional view of the angular rate sensor accordingto the modification of the first embodiment taken along line A-A in FIG.1A;

FIG. 7A and FIG. 7B show an angular rate sensor according to the secondembodiment, wherein FIG. 7A shows a bird's-eye view of the angular ratesensor according to the second embodiment and FIG. 7B shows across-sectional view taken along B-B line in FIG. 6A;

FIG. 8 shows a cross sectional view illustrating a snap shot ofvibrating components of the angular rate sensor according to the secondembodiment;

FIG. 9 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 7A and FIG. 7B according to the second embodimentof the present invention;

FIGS. 10A to 10D illustrate the steps performed in the method formanufacturing the angular rate sensor according to the secondembodiment;

FIG. 11 shows a cross-sectional view of the angular rate sensoraccording to the modification of the second embodiment taken along lineB-B in FIG. 7A;

FIG. 12 shows a cross-sectional view of the angular rate sensoraccording to the third embodiment;

FIG. 13 shows a cross sectional view illustrating a snap shot ofvibrating components of the angular rate sensor according to the thirdembodiment;

FIG. 14 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 12 according to the third embodiment of the presentinvention;

FIGS. 15A to 15E illustrate the steps performed in the method formanufacturing the angular rate sensor according to third embodiment;

FIG. 16 shows a cross-sectional view of the angular rate sensoraccording to the modification of the third embodiment;

FIG. 17A and FIG. 17B show an angular rate sensor according to the forthembodiment, wherein FIG. 17A shows a bird's-eye view of the angular ratesensor according to the forth embodiment and FIG. 14B shows across-sectional view taken along C-C line in FIG. 17A;

FIG. 18 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 17A and FIG. 17B according to the forth embodimentof the present invention;

FIGS. 19A to 19D illustrate the steps performed in the method formanufacturing the angular rate sensor according to the forth embodiment;

FIG. 20A and FIG. 20B show an angular rate sensor 1 according to thefifth embodiment, wherein FIG. 20A shows a bird's-eye view of theangular rate sensor 1 and FIG. 20B shows a cross-sectional view takenalong A-A line in FIG. 20A;

FIG. 21 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 20A and FIG. 20B according to the fifth embodimentof the present invention;

FIGS. 22A to 22D illustrate the steps performed in the method formanufacturing the angular rate sensor according to the fifth embodiment;

FIG. 23 shows a cross-sectional view of the angular rate sensoraccording to the modification of the fifth embodiment;

FIG. 24 shows a cross-sectional view of the angular rate sensoraccording to the sixth embodiment;

FIG. 25 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 22A and FIG. 22B according to the sixth embodimentof the present invention;

FIGS. 26A to 26E illustrate the steps performed in the method formanufacturing the angular rate sensor according to the sixth embodiment;

FIG. 27 shows a cross-sectional view of the angular rate sensoraccording to the modification of the sixth embodiment;

FIGS. 28A and 28B show angular rate sensors according to themodifications of the sixth embodiment;

FIG. 29A and FIG. 29B show an angular rate sensor 1 according to theseventh embodiment, wherein FIG. 29A shows a bird's-eye view of theangular rate sensor and FIG. 29B shows a cross-sectional view takenalong B-B line in FIG. 29A;

FIG. 30 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 29A and FIG. 29B according to the seventhembodiment of the present invention;

FIGS. 31A to 31G illustrate the steps performed in the method formanufacturing the angular rate sensor according to the seventhembodiment;

FIG. 32 shows a cross-sectional view of the angular rate sensoraccording to the modification of the seventh embodiment;

FIG. 33A and FIG. 33B show an angular rate sensor according to theeighth embodiment, wherein FIG. 33A shows a bird's-eye view of theangular rate sensor and FIG. 33B shows a cross-sectional view takenalong C-C line in FIG. 33A;

FIG. 34 is a flow chart of steps for manufacturing the angular ratesensor shown in FIG. 33A and FIG. 33B according to the eighth embodimentof the present invention;

FIGS. 35A to 35G illustrate the steps performed in the method formanufacturing the angular rate sensor according to the eighthembodiment;

FIG. 36 shows a cross-sectional view of the angular rate sensoraccording to the first modification of the eighth embodiment;

FIG. 37 shows a cross-sectional view of the angular rate sensoraccording to the second modification of the eighth embodiment;

FIG. 38 shows a cross-sectional view of the angular rate sensoraccording to the third modification of the eighth embodiment;

FIG. 39A and FIG. 39B show an angular rate sensor according to the ninthembodiment, wherein FIG. 39A shows a bird's-eye view of the angular ratesensor and FIG. 39B shows a cross-sectional view taken along A-A line inFIG. 39A;

FIG. 40 shows a bird's-eye view of the first modification of the ninthembodiment;

FIG. 41A and FIG. 41B show an angular rate sensor according to the firstmodification of the ninth embodiment, wherein FIG. 41A shows a top viewof the angular rate sensor and FIG. 41B shows a cross-sectional viewtaken along B-B line in FIG. 41A;

FIG. 42A and FIG. 42B show an angular rate sensor according to thesecond modification of the ninth embodiment, wherein FIG. 42A shows atop view of the angular rate sensor and FIG. 42B shows a cross-sectionalview taken along C-C line in FIG. 42A;

FIG. 43 shows an angular rate sensor according to the third modificationof the ninth embodiment;

FIG. 44A and FIG. 44B show an angular rate sensor according to the forthmodification of the ninth embodiment, wherein FIG. 44A shows abird's-eye view of the angular rate sensor and FIG. 44B shows a topview;

FIG. 45 shows a known prior art embodiment of an angular rate sensorutilizing a single crystalline piezoelectric element;

FIG. 46A and FIG. 46B show the individual axes of a quartz crystal;

FIG. 47 is a diagram of a prior art embodiment of an angular ratesensor;

FIG. 48 shows a simplified operating principle of the above-mentionedangular rate sensor shown in FIG. 47; and

FIG. 49 shows a cross sectional view illustrating a snap shot ofvibrating components of the angular rate sensor according to the priorart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to describe the present invention in more detail, the variousembodiments of the present invention will now be described hereafterwith references to accompanying drawings.

First Embodiment

Description of a first embodiment of the present invention will be madewith reference to FIGS. 1 to 6.

FIG. 1A and FIG. 1B show a structure of an angular rate sensor 1according to a first embodiment of the present invention. FIG. 1A is abird's-eye view of the angular rate sensor 1 and FIG. 1B is across-sectional view thereof taken along line A-A in FIG. 1A.

As shown in FIG. 1B, the angular rate sensor 1 includes a semiconductorsubstrate 2 having an upper surface. The semiconductor substrate is madeof, for example, silicon. It is preferable that the thickness of thesemiconductor substrate is larger than 400 μm. The angular rate sensor 1has sensing components disposed on the upper surface of thesemiconductor substrate 2.

An thin insulator film 3 is formed on the upper surface of thesemiconductor substrate 3 so as to cover a whole surface of thesemiconductor substrate 3. The thin insulator film is made of siliconoxide (SiO₂), for example. The thickness of the thin insulator film isthicker than 1 μm. On an opposite surface of the thin insulator film tothat connecting to the semiconductor substrate 3, a first electrode 4 ofseveral hundreds nanometer in thickness is formed. The first electrode 4is composed of one of impurity doped poly-silicon, aluminum (Al),aluminum-base alloy, titanium (Ti), titanium-base alloy, tungsten,tungsten-base alloy, molybdenum, and molybdenum-base alloy with thesemiconductor production technology for very-large-scale integratedcircuits (VLSIs) while taking into account of an environmental metalpollution.

A piezoelectric film 5 is disposed on the first electrode 4 so as tocover the whole surface of the first electrode 4. The thickness of thepiezoelectric film 5 is about several micrometers. For example, thepiezoelectric film 5 is made of aluminum nitride (AlN) or zinc oxide(ZnO), but also of a piezoelectric and ferroelectric film such asPTZ:Pb(ZrTi)O₃, PT:PbTiO₃, and the like. In the case where thepiezoelectric film is made of aluminum nitride (AlN), another functionaldevice, such as a complementary metal-oxide-semiconductors (CMOS), ispossible to integrate into the angular rate sensor while the issue ofenvironmental metal pollution is taken into account.

Further, perturbation masses 6, driving electrodes 7-10, reflectors 11,12, and detecting electrodes 13-16 are disposed on the piezoelectricfilm 5. A plurality of the driving electrodes constructs a drivinginter-digital transducer (hereinafter, “driving IDT”). Similarly, aplurality of the detecting electrodes constructs a detectinginter-digital transducers (hereinafter, “detecting IDTs”)

The driving electrodes 7-10 are used to excite an elastic acoustic wavein order to cause a standing wave on which a Coriolis force acts whenthe angular rate sensor is rotated. Each of the driving electrodes 7-10is constructed by a comb-shaped electrode. The driving electrodes 7-8and the driving electrodes 9-10 are separated at predetermined spacedefined by a frequency of the standing wave, respectively. Each of thedetecting electrodes 13-16 is also constructed by a comb-shapedelectrode. The reflectors 11, 12 serves to repeatedly reflect theelastic acoustic waves caused by the driving IDTs 13-16 so as to excitethe standing wave between the reflectors 11, 12.

The perturbation masses 6, the driving electrodes 7-10, the reflectors11, 12, and the detecting electrodes 13-16, are made of one of impuritydoped poly-silicon, aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy. If the driving electrodes 7-10, the reflectors11, 12, and the detecting electrodes 13-16, are made of any one ofimpurity doped poly-silicon, aluminum (Al), and Al—Si—Cu with thesemiconductor production technology for VLSIs, it is not necessary totake into account the environmental metal pollution during manufacturingthe angular rate sensor. If the perturbation mass 6 is made of metals ormetallic alloys including platinum (Pt), tungsten (W), and gold (Au). Insuch a configuration of the perturbation masses 6, the Coriolis forceacting on the perturbation masses 6 is emphasized so as to increase thedisplacement of the perturbation masses 6. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.

As shown in FIG. 1A, the perturbation masses 6 are composed of metallicdots and are scattered over a predetermined region 60 on thepiezoelectric film 5. Each of the perturbation masses 6 are formed inthe shape of a square as seen from the top view. The perturbation masses6 are arrayed so as to from a staggered lattice which is covered by thepredetermined region 60.

More concretely, consider that the perturbation masses 6 lie in an x-yplane, in which the x-axis runs from the reflector 11 to another one 12,and the y-axis runs from the detecting electrodes 13, 14 to another ones15, 16. The perturbation masses 6 are interlaced in both the x-directionand y-directions such that the elastic acoustic waves generated by theCoriolis force are coherently superposed. When the Coriolis force actson the perturbation masses 6, the perturbation masses which are inalignment along the y-direction vibrate coherently and the perturbationmasses 6 in the neighboring row in the x-direction vibrate coherentlywith the inverse phase since the perturbation masses 6 is located fromevery metallic dots by λ₁/2 in the x-direction and by λ₂/2 in they-direction, where λ₁ is a wavelength of the standing wave caused by thedriving electrodes 7-10 and organized by the reflectors 11, 12, and λ₂is a wavelength of an elastic acoustic wave generated by the Coriolisforce acting on the perturbation masses 6. So, when the angular ratesensor 1 rotates about the x-axis with rotation rate

, then each of the perturbation masses 6 experiences an acceleration of2

×

_(x), in the y-direction, where

is the velocity vector of particle on which the individual perturbationmasses 6 is disposed.

The driving electrodes 7-10 are arranged on the piezoelectric film 5 sothat the predetermined region 60 is sandwiched between the drivingelectrodes 7-8 and the driving electrodes 9-10 in the x-direction and issandwiched between the detecting electrodes 13-14 and the detectingelectrodes 15-16 in the y-direction. The driving electrodes 7-10 areconnected to the electric power supply (not shown) via driving-voltagesupply lines consisting of bonding wires and the like, and are suppliedelectric driving voltage from the electric power supply. Driving voltageis applied between the driving electrodes 7-10 and the first electrode 4so as to excite vibration of the driving electrodes 7-10 in thez-direction, i.e., in the perpendicular direction to the surface of thepiezoelectric film 5, as in FIGS. 1A and 1B due to the piezoelectriceffect by which the electric power is converted into the mechanicaldeformation energy.

The driving electrodes 7, 8 are coupled for constituting a first drivinginter-digital transducer (driving IDT). Each driving electrodes 7, 8 isformed in a comb-shape, that is, the driving electrodes 7, 8 includetooth components 7 a, 8 a, which are disposed to be parallel to they-axis and connecting components 7 b, 8 b, which are disposed to beperpendicular to the y-direction and serves as connectors of a pluralityof the tooth components 7 a, 8 a. The first driving IDT composed of thedriving electrodes 7, 8, is positioned on a side of the predeterminedregion 60 on which the perturbation masses 6 are disposed. In thedriving electrodes 7, 8, the tooth components 7 a, 8 a, are arranged tobe interlaid with each other. The connecting components 7 b, 8 b, arelocated face to face across the tooth components 7 a, 8 a. A spacing oftooth components 7 a, 8 a, of the driving electrodes 7, 8, in thex-direction determines a wavelength of an elastic acoustic wavegenerated by the first driving IDTs 7, 8, themselves.

Similarly to the driving electrodes 7, 8, the driving electrodes 9, 10are coupled for constituting a second driving inter-digital transducer(driving IDT). The second driving IDT composed of detecting electrodes9, 10, is positioned on an opposite side of the predetermined region 60to the first driving IDT composed of the detecting electrodes 7, 8. Eachdriving electrodes 9, 10 is formed in a comb-shape. The drivingelectrodes 9, 10 include tooth components 9 a, 10 a, which are disposedto be parallel to the y-axis and connecting components 9 b, 10 b, whichare perpendicular to the y-direction. The connecting components 9 b, 10b bridge the tooth components 9 a, 10 a, thereby. As is the case of thedriving electrodes 7, 8, the tooth components 9 a, 10 a, are arranged tobe interlaid with each other and located to be spaced apart with aperiodicity of one half the wavelength of the elastic acoustic wavegenerated by the driving IDTs 7-10 themselves.

The reflectors 11, 12 are disposed on the surface of the piezoelectricfilm 5 such that the perturbation masses 6 and the driving electrodes7-10 are located between the reflector 11 and 12 along the x-direction.The reflectors 11, 12 are disposed on the piezoelectric film 5 andfabricated in a rod shape. A longitudinal direction of both reflectors11, 12 runs parallel to the y-axis, that is, to the tooth components 7a-10 a of the driving electrodes 7-10.

The detecting electrodes 13, 14 are coupled in order to contribute afirst detecting inter-digital transducer (detecting IDT). The detectingelectrodes 13, 14 are located on the piezoelectric film 5. Eachdetecting electrodes 13, 14 is formed in a comb-shape. The detectingelectrodes 13, 14 include tooth components 13 a, 14 a, which aredisposed to be parallel to the x-axis and further include connectingcomponents 13 b, 14 b, which are perpendicular to the x-direction. Theconnecting components 13 b, 14 b bridge the tooth components 13 a, 14 a,thereby. The tooth components 13 a, 14 a, are arranged to be interlaidwith each other and located to be spaced apart with a periodicity of onehalf the wavelength of the elastic acoustic wave generated by theCoriolis force.

Similarly to the detecting electrodes 13, 14, the detecting electrodes15, 16 are coupled in order to constitute a second detectinginter-digital transducer (detecting IDT). The detecting electrodes 15,16 are located on the piezoelectric film 5. Each detecting electrodes15, 16 is formed in a comb-shape. The detecting electrodes 15, 16include tooth components 15 a, 16 a, which are disposed to be parallelto the y-axis and further include connecting components 15 b, 16 b,which are perpendicular to the y-direction. The connecting components 15b, 16 b bridge the tooth components 15 a, 16 a, thereby. The toothcomponents 15 a, 16 a, are arranged to be interlaid with each other andlocated to be spaced apart with a periodicity of one half the wavelengthof the elastic acoustic wave generated by the Coriolis force.

The perturbation masses 6, the driving electrodes 7-10, reflectors 11,12, the detecting electrodes 13-16 consist of a sensing unit of theangular rate sensor 1 according to the first embodiment of the presentinvention.

In the following, the operations of the angular rate sensor 1 accordingto the first embodiment of the present invention will be described.

As shown in FIG. 2, if the angular rate sensor 1 according to the firstembodiment of the present invention undergoes a rotary motion while anelastic acoustic waves caused at least in the piezoelectric film 5 isgenerated by the driving electrodes 7-10, the rotation perpendicular tothe vibrating velocity of each of the perturbation masses 6 produces theCoriolis force in the direction perpendicular to both directions. Theelastic acoustic wave is sometimes caused in the first electrode 4, anda surface of the semiconductor substrate in addition to thepiezoelectric film 5. Generally, the elastic acoustic wave penetrates anelastic material in order of one wave length depth from the drivingelectrodes 7-10.

In the angular rate sensor 1 according to this embodiment, the sensingunit is driven by applying a driving voltage to the driving electrodes7-10. For example, an alternating current (AC) voltage +B[V] is suppliedto the driving electrodes 7 and 9, whereas an alternating current (AC)voltage −B[V] is applied to the driving electrodes 8 and 10. In otherwords, the phase of the AC voltage applied to the driving electrodes 7and 9 is different by a half of the periodicity of the driving ACvoltage from that applied to the driving electrodes 8 and 10. Ifnecessary, the first electrode 4 is established a ground so that adifference in an electric potential between the driving electrodes 7-10and the first electrode 4 is generated. This means that electric fieldsexist in the piezoelectric film 5 which has the lower surface coveringthe first electrode 4 and has the upper surface on which the drivingelectrodes 7-10 are disposed. By the piezoelectric effect, the drivingelectrodes 7-10 are vibrated in the z-direction so that elastic acousticwaves are caused and propagated along the x-direction.

The operating frequency of the angular rate sensor 1 is determined bythe separation of the driving electrodes 7-10, physical characteristicsof the piezoelectric film 5 and the semiconductor substrate 2, forexample, a silicon substrate and so on. The driving electrodes 7-10generate an elastic acoustic wave of the piezoelectric film having afrequency ranging from several MHz to several hundreds MHz.

In contrast to a usual case where a propagating wave is generated in atransmission channel, the driving electrodes 9, 10 and the reflectors11, 12 reflect the propagating wave generated by the driving electrodes7-10 and confine the propagating wave into the transmission channel toestablish a first standing wave in the piezoelectric film. The pluralityof the perturbation masses 6 is arranged to locate at the anti-nodalpoints of this standing wave in order to amplify the magnitude of theCoriolis force to excite an elastic acoustic wave orthogonal to bothdirections of the first standing wave and the Coriolis force andtherefore to improve the sensitivity of the angular rate sensor.

If each perturbation mass 6 has a mass, m, and a velocity of eachperturbation mass 6,

, when the angular rate sensor 1 undergoes a rotary motion with anangular rotation

perpendicular to the direction of the velocity of each perturbation mass6.

causes the Coriolis force

perpendicular to the directions of both the vibrating motion

and the angular rotation

. Where the velocity of each perturbation mass 6,

, and the angular rotation

. Thus, the effect of the Coriolis force is written in a form,

=2m

×

.

Where a symbol “x” in the above equation represents an external product.If it is assumed that the resonant frequency of the first standing waveis

(≡2πf₀) and the amplitude of the first standing wave is written by r,thus, the velocity of each perturbation mass 6,

can be written by

=r

.

In this case, the elastic acoustic wave generated by the drivingelectrodes 7-10 is rectified by reflecting back and forward between thereflectors 11, 12, so as to synchronize the frequency of the firststanding wave with that of the of the externally applied Ac voltage.

Since the perturbation masses 6 are arranged to form a staggered latticeso as to act the Coriolis force effectively, a coherent alternatingforce generated at each perturbation mass 6 by the Coriolis forcecreates another vibrating motion of each perturbation mass 6 in they-direction. In other words, the Coriolis force generates a secondpropagating wave in the piezoelectric film 5 along the y-direction. Inthe y-direction, the detecting electrodes 13-16 are positioned at bothsides of the predetermined region 60 on which the perturbation masses 6are disposed. The second propagating wave is sensed by the detectingelectrodes 13-16 as an electric signal converted due to thepiezoelectric effect. Therefore, the angular rotation

can be obtained.

In the angular rate sensor 1 according to this embodiment, the firstelectrode 4 is formed so as to be covered by the piezoelectric film 5.Above a region on which the first electrode 4 is formed, thepredetermined region 60 on which the perturbation masses 6 are disposedis located.

FIG. 3 shows a cross-sectional view of the angular rate sensor 1according to this embodiment taken along line A-A in FIG. 1A while thefirst standing wave is excited in the piezoelectric film 5 so that astress force is generated in the piezoelectric film 5 due to thepiezoelectric effect. Due to this stress force generated in thepiezoelectric film 5, a polarization in the piezoelectric film 5 alongthe z-direction is exhibited such that positively or negatively chargedregions near the upper surface of the piezoelectric film 5 iappearswherein a sign of the electric charge is depended on a piezoelectricconstant of the material making the piezoelectric film 5. An oppositelycharged region also appears near the lower surface of the piezoelectricfilm 5.

However, the angular rate sensor 1 according to this embodiment includesthe first electrode 4 on which the piezoelectric film 5 is formed. Thus,near the lower surface of the piezoelectric film 5, the polarizationexhibited due to the first standing wave in the piezoelectric film isneutralized via the first electrode 4. Therefore, even if surfaceelectric charge is generated at the lower surface of the piezoelectricfilm 5 connecting to the first electrode 4, surface electric chargeescapes from the piezoelectric film 5 and thus is neutralized via thefirst electrode 4. Therefore, limitations of an amount of a displacementof the vibrating particle in the piezoelectric film 5 and eachperturbation mass 6 vibrating in the z-direction due to the polarizationin the z-direction of the piezoelectric film 5 are eliminated.

When the angular rate sensor 1 undergoes a rotary motion with an angularrotation

under which each perturbation mass 6 has a vibrating velocity vector V,the Coriolis force whose amplitude is proportional to the vibratingvelocity

acts on particles in the piezoelectric film 5 and the perturbationmasses 6. Thus, an addition to the limitations of an amount of adisplacement of the vibrating particle in the piezoelectric film 5 andeach perturbation mass 6 vibrating in the z-direction due to thepolarization in the z-direction of the piezoelectric film 5, limitationsof a vibrating velocities of the vibrating particle in the piezoelectricfilm 5 and each perturbation mass 6 are removed. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.

A method for fabricating the angular rate sensor 1 with productiontechnology for very-large-scale integrated circuits (VLSIs) will bedescribed.

FIG. 4 shows a flow chart of steps for manufacturing the angular ratesensor 1 includes a preparing step (S100) for preparing thesemiconductor substrate 2, a first electrode forming step (S102) fordepositing the first electrode 3 on the semiconductor substrate 2 atleast over a region above which perturbation masses 6 is formed, apiezoelectric film forming step (S104) for depositing the piezoelectricfilm 5 on at least one of the first electrode 3 and the semiconductorsubstrate 2, and a fabricating step (S106) for fabricating a pluralityof features on the piezoelectric film 5, wherein the plurality offeatures includes the perturbation masses 6, a driving electrode 7-10for causing a elastic acoustic wave in the piezoelectric film 5,reflectors 11, 12 for reflecting the elastic acoustic wave so as to forma first and a second standing wave, and detecting electrodes 13-16 fordetecting a second standing wave caused by the Coriolis force acting onthe first standing wave with production technology for very-large-scaleintegrated circuits (VLSIs).

FIGS. 5A to 5C illustrate the steps performed in the method formanufacturing the angular rate sensor 1.

First, a (100)-oriented silicon substrate of 400 μm thickness isprepared (S100 in FIG. 4). The thickness of a silicon substrate is notso important, therefore silicon substrate being thicker than 400 μm isnot forbidden. As for the substrate, it is possible to use not only asingle crystal substrate made of single crystal such as silicon orsapphire, but also a substrate other than a single crystal, such as aglass substrate, a polycrystalline ceramic substrate, a metal substrateor a resin substrate. Using the semiconductor substrate such as siliconhas an advantage that it is possible to integrate the angular ratesensor 1 and an external driving circuit thereof into a single device sothat a large-scale integration can be achieved.

Next, as shown in FIG. 4A, a thin silicon dioxide film which is thinnerthan 1 μm is grown on the silicon substrate. The silicon dioxide film isformed, for example, by sputtering.

As shown in FIG. 4B, a several hundreds nanometer thick film ofimpurity-doped polycrystalline silicon is deposited by low pressurechemical vapor deposition (LPCVD) for forming a first electrode 4 (S102in FIG. 4). For example, phosphor-doped polycrystalline silicon is usedfor the first electrode 4. In this case, phosphor-doped polycrystallinesilicon film is patterned using reactive ion etching technique with aflouride-based gas. If the first electrode 4 is composed of one ofaluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy,sputtering technique is suitable one for depositing the first electrode4 on the semiconductor substrate 3.

In the next step, a several micrometer thick film of a piezoelectricfilm such as aluminum nitride (AlN), zinc oxide (ZnO), zirconate titante(PZT), lead titanate (PT), lithium tantalite (LiTaO₃), and lithiumtantalite (LT), is disposed so as to cover the whole region of an uppersurface of the first electrode 4 (S104 in FIG. 4). If an aluminumnitride (AlN) film is used as the first electrode 4, the AlN film ispatterned by a tetra methyl ammonium hydroxide-based solvent.

Then, as shown in FIG. 4C, a several hundreds thick film of metal orconductive metallic alloy including aluminum (Al), aluminum-base alloy,titanium (Ti), titanium-base alloy, tungsten, tungsten-base alloy,molybdenum, and molybdenum-base alloy, is deposited using an e-beamevaporator, and patterned by a phosphoric (H₃PO₄)-based acid to form theperturbation masses 6, the driving electrodes 7-10, reflectors 11, 12,and the detecting electrodes 13-16 on an upper surface of thepiezoelectric film 5 (S105 in FIG. 4). Further, the supplying drivingvoltage lines and ground lines are formed and connected to the drivingelectrodes 7-10, reflectors 11, 12, and the detecting electrodes 13-16by wire bonding.

In the angular rate sensor 1 having the first electrode 4 sandwichedbetween the semiconductor substrate 3 and the piezoelectric film 5, thepolarization in the z-direction can be suppressed even if an innerstress in the piezoelectric film 5 is generated by the Coriolis forceacting on vibrating particles in the piezoelectric film 5 and then theinner stress generates an electric field in the z-direction which is aorigin of a surface electric charge, i.e., the electric polarization ofthe piezoelectric film 5 in the z-direction due to neutralization effectby the first electrode 4. In more detail, if surface electric charge isgenerated at the lower surface of the piezoelectric film 5 connecting tothe first electrode 4, surface electric charge escapes from thepiezoelectric film 5 and thus is neutralized via the first electrode 4.Therefore, a limitation of an amount of a displacement of the vibratingparticle in the piezoelectric film 5, especially of the perturbationmasses 6 vibrating in the z-direction due to the polarization in thez-direction of the piezoelectric film 5 is eliminated. In consequence,it is possible to realize an angular rate sensor having highsensitivity.

Further, the angular rate sensor 1 according to this embodiment, thefirst electrode 4 is formed below the driving electrodes 7-10 via thepiezoelectric film 5. Therefore, there exits electric fields between thedriving electrodes 7-10 and the first electrode 4 generated by theexternally driving AC voltage so that it is possible to vibrate thedriving electrodes 7-10 with a larger amplitude in the z-direction. Inconsequence, it is possible to realize an angular rate sensor havinghigh sensitivity.

Further, a basic structure of the angular rate sensor 1 according tothis embodiment on which the driving electrodes 7-10, reflectors, 11,12, and the detecting electrodes 13-16 take, resembles that taken by theprior arts. Hence, it becomes possible to reduce a size of the angularrate sensor because a necessary size for obtaining a sufficientsensitivity becomes smaller due to higher sensitivity of the angularrate sensor 1 according to this embodiment. Therefore the angular ratesensor having high sensitivity can be miniaturized with productiontechnology for very-large-scale integrated circuits (VLSIs).

Further it is preferable that the perturbation masses 6 are made ofmetal or metallic alloy whose mass density is greater than 13.5 g/cm³.The metals or metallic alloys being suitable ones for the perturbationmasses or the perturbation weight include platinum (Pt), tungsten (W),and gold (Au). In such a configuration of the perturbation masses 6, theCoriolis force acting on the perturbation masses 6 is emphasized so asto increase the displacement of the perturbation masses 6 t in thez-direction. In consequence, it is possible to realize an angular ratesensor having high sensitivity. Further the angular rate sensor havinghigh sensitivity can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

Further it is easy to be recommended for size reduction of the device orintegration an angular rate sensor 1 and an external driving circuitthereof into an integrated device since the angular rate sensor 1according to this embodiment use the semiconductor substrate 2 such assilicon substrate.

Modification of First Embodiment

FIG. 6 shows a cross-sectional view of the angular rate sensor 1according to the modification of the first embodiment taken along lineA-A in FIG. 1A.

In the first embodiment described above in which the perturbation masses6, the driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are disposed directly on the upper surface of thepiezoelectric film 5, if necessary, an thin insulator film 20 is formedand inserted between the upper surface of the piezoelectric film 5 andthe perturbation masses 6, the driving electrodes 7-10, the reflectors11, 12, and the detecting electrodes 13-16. In this configuration, theperturbation masses 6, the driving electrodes 7-10, the reflectors 11,12, and the detecting electrodes 13-16 are disposed on the thininsulator film 20. It is especially preferable in a case where thepiezoelectric film 5 is made of AlN that the thin insulator film 20 isformed on the supper surface of the piezoelectric film 5 in order tosuppress a leakage electric current flowing along the z-direction in thepiezoelectric film 5 from the driving electrodes 7-10 toward the firstelectrode 4, when the driving AC voltage is supplied between the drivingelectrodes 7-10 and the first electrode 4.

Second Embodiment

Referring to FIGS. 7A-11, an angular rate sensor according to the secondembodiment of the present invention will now be explained. In thissecond embodiment, the identical components in structure to those in thefirst embodiment are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

The angular rate sensor according to the second embodiment has astructure capable of improving a neutralization effect of the surfacecharge generated at the upper and lower surfaces of the piezoelectricfilm 5 while the elastic acoustic wave is excited in the piezoelectricfilm 5 by the applying the driving AC voltage between the drivingelectrodes 7-10 and the first electrode 4. Thus, since the operatingprinciple of the angular rate sensor is the same to that of the angularrate sensor according to the first embodiment, the difference of theangular rate sensor according to this embodiment from that according tothe first embodiment will be described.

FIG. 7A and FIG. 7B show an angular rate sensor 1 according to thesecond embodiment. FIG. 7A shows a bird's-eye view of the angular ratesensor 1 and FIG. 7B shows a cross-sectional view taken along B-B linein FIG. 7A.

As shown in FIG. 7A, an angular rate sensor in this embodiment furthercomprises a second electrode 30 disposed on the upper surface of thepiezoelectric film 5. This configuration is enabled to neutralize asurface charge generated at the upper surface of the piezoelectric film5. In more detail, the second electrode 30 is disposed on thepiezoelectric film 5 so as to cover a predetermined region 60 on whichthe perturbation masses 6 are formed.

FIG. 8 shows a cross sectional view illustrating a snap shot ofvibrating components of the angular rate sensor 1 when the angular ratesensor 1 undergoes a rotary motion after the first standing wave iscaused at least in the piezoelectric film 5.

In the angular rate sensor 1 having the second electrode 30 sandwichedbetween the piezoelectric film 5 and the perturbation masses 6, thepolarization in the z-direction can be suppressed due to neutralizationeffect by the second electrode 30 even if an inner stress in thepiezoelectric film 5 is generated by the Coriolis force acting onvibrating particles in the piezoelectric film 5. The inner stressgenerates an electric field in the z-direction which causes a surfaceelectric charge at an upper and a lower surface of the piezoelectricfilm or an electric polarization of the piezoelectric film in thez-direction. In more detail, if surface electric charge is generated atthe upper surface of the piezoelectric film 5 connecting to the secondelectrode 30, surface electric charge escapes from the piezoelectricfilm 5 and thus is neutralized via the second electrode 30. Therefore, alimitation of an amount of a displacement of the vibrating particle inthe piezoelectric film 5, especially of the vibrating perturbationmasses 6 in the z-direction due to the polarization in the z-directionof the piezoelectric film 5, is eliminated. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.Further the angular rate sensor having high sensitivity can beminiaturized with production technology for very-large-scale integratedcircuits (VLSIs).

A method for fabricating the angular rate sensor 1 according to thesecond embodiment with production technology for very-large-scaleintegrated circuits (VLSIs) will be described.

FIG. 9 shows a flow chart of steps for manufacturing the angular ratesensor 1 includes a preparing step (S200) for preparing thesemiconductor substrate 2, a first electrode forming step (S202) fordepositing the first electrode 3 on the semiconductor substrate 2 atleast over a region above which perturbation masses 6 is formed, apiezoelectric film forming step (S204) for depositing the piezoelectricfilm 5 on at least one of the first electrode 3 and the semiconductorsubstrate 2, a second electrode forming step (S206) for depositing thesecond electrode 30 on the upper surface of the piezoelectric film 5,and a fabricating step (S208) for fabricating a plurality of features onthe piezoelectric film 5, wherein the plurality of features includes theperturbation masses 6, a driving electrode 7-10 for causing an elasticacoustic wave in the piezoelectric film 5, reflectors 11, 12 forreflecting the elastic acoustic wave so as to form a first and a secondstanding wave, and detecting electrodes 13-16 for detecting a secondstanding wave caused by the Coriolis force acting on the first standingwave with production technology for very-large-scale integrated circuits(VLSIs).

FIGS. 10A to 10D illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as in the method for manufacturing the angular rate sensoraccording to the first embodiment, a 400 μm or the more thick(100)-oriented silicon substrate is prepared (S200 in FIG. 9).

Next, as shown in FIG. 10A, a thin silicon dioxide film which is thinnerthan 1 μm is grown on the silicon substrate in the same way shown inFIG. 4A.

As shown in FIG. 10B, a several hundreds nanometer thick film ofimpurity-doped polycrystalline silicon is deposited in the same wayshown in FIG. 4B (S202 in FIG. 9). This thick film becomes to a firstelectrode 4. The first electrode 4 has an upper surface which is anopposite surface connecting the silicon substrate.

In the next step, a several micrometer thick film of a piezoelectricfilm such as aluminum nitride (AlN), zinc oxide (ZnO), zirconate titante(PZT), lead titanate (PT), lithium tantalite (LiTaO₃), and lithiumtantalite (LT), is disposed so as to cover the whole region of an uppersurface of the first electrode 4 (S204 in FIG. 9). If an aluminumnitride (AlN) film is used as the first electrode 4, the AlN film ispatterned by a tetra methyl ammonium hydroxide-based solvent.

Then, as shown in FIG. 10C, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited on the piezoelectric film 5 using ane-beam evaporator, and patterned by a phosphoric (H₃PO₄)— based acid toform the second electrode 30. Thus, the second electrode 30 is formed(S206 in FIG. 9). The second electrode 30 has an upper surface which isan opposite surface connecting with the piezoelectric film 5.

Next, as shown in FIG. 10D, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the perturbationmasses 6, the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 on an upper surface of the piezoelectric film5 (S208 in FIG. 9), in the same way shown in FIG. 4C.

In this step, it is preferable that a selective etching technique havingthe proper etching ratio of one material of the second electrode 30 toanother material of the perturbation masses 6 and the driving electrodes7-10, is used. If it is not possible to use the selective etchingtechnique, etching time is adjusted to prevent etching the secondelectrode 30 while the thin film of metal or conductive metallic alloyis patterned to form the perturbation masses 6 and the drivingelectrodes 7-10.

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Second Embodiment

In the second embodiment described above in which the second electrode30, the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16 are disposed directly on the upper surface ofthe piezoelectric film 5.

As shown in FIG. 11, if necessary, a thin insulator film 20 is formedand inserted between the upper surface of the piezoelectric film 5.Consequently, the second electrode 30, the driving electrodes 7-10, thereflectors 11, 12, and the detecting electrodes 13-16 are formed on thethin insulator film 20. In this configuration, the second electrode 30,the driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are disposed on the thin insulator film 20. In thisconfiguration, it is possible to prevent etching the piezoelectric film5 while the thin film of metal or conductive metallic alloy is patternedto form the second electrode 30, the perturbation masses 6, and thedriving electrodes 7-10.

Third Embodiment

Referring to FIGS. 12-16, an angular rate sensor according to the thirdembodiment of the present invention will now be explained. In thisembodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

The angular rate sensor 1 according to the third embodiment, includesthe semiconductor substrate 2 such as silicon substrate, the insulatorfilm 3 such as silicon dioxide film disposed on the semiconductorsubstrate 2, the first electrode 4 disposed on the first insulator film3, the piezoelectric film 5 disposed on the first electrode 4, thesecond insulator film 20 disposed on the piezoelectric film 5, thesecond electrode 30 disposed on the second insulator film 20 on whichthe perturbation masses 6 are located, the driving electrode 7-10 forcausing a first elastic acoustic wave in the piezoelectric film 5, thereflectors 11, 12 for reflecting the elastic acoustic wave caused by thedriving electrodes 7-10 to form a standing wave of the elastic acousticwave in the piezoelectric film 5, the detecting electrodes 13-16 fordetecting a second elastic wave generated by the Coriolis force actingon the perturbation masses 6, wherein the driving electrodes 7-10, thereflectors 11, 12, the detecting electrodes 13-16 are formed on thesecond insulator film 20, as for the angular rate sensor according tothe second embodiment.

The angular rate sensor 1 according to the third embodiment furthercomprises a contact hole 5 a formed in the piezoelectric film 5 forelectrically connecting the first electrode 4 sandwiched between thefirst insulator film 3 and the piezoelectric film 5 to the secondelectrode 30 formed on the second insulator film 20 but below theperturbation masses 6 so as to keep the same electric potential of thefirst electrode 4 with that of the second electrode 30.

FIG. 12 shows a cross-sectional view of the angular rate sensoraccording to the third embodiment of the present invention. As shown inFIG. 12, the angular rate sensor has a plurality of contact holes 5 a isformed in the piezoelectric film 5 such that the perturbation masses 6are located on the individual top of the contact holes 5 a. For example,each contact holes 5 a is in shape of a pole.

Even if the contact holes 5 a are absent. the second electrode 30 formedon the piezoelectric film 5 is configured to neutralize the surfacecharged generated at the upper surface of the piezoelectric film 5 dueto the piezoelectric effect. However, the first electrode 4 and thesecond one 30 are not directly connected and are separated through thepiezoelectric film 5, a difference in electric potential between thefirst and second electrodes 5, 30 may be appeared so that electricfields are generated by the surface charges at upper and lower surfacesof the piezoelectric film. The contact holes 5 a improves such thesituation by keeping the electric potentials of the first and secondelectrodes 5, 30 at the same level and preventing for appearing thedifference of the electric potentials of the first and second electrodes5, 30. Therefore, a limitation of an amount of a displacement of thevibrating particle in the piezoelectric film 5, especially of thevibrating perturbation masses 6 in the z-direction due to thepolarization in the z-direction of the piezoelectric film 5, iseliminated. In consequence, it is possible to realize an angular ratesensor having high sensitivity. Further the angular rate sensor havinghigh sensitivity can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

Further, in the angular rate sensor 1 of this type, it is preferablethat the each contact hole 5 a is connecting to the correspondingperturbation mass 6. In other words, the individual perturbation masses6 are formed on the top of the contact hole 5 a, as shown in FIG. 13. Ifthe contact holes 5 a are formed at the position where the perturbationmasses 6 are disposed, some portion of a weight of the contact holes 5 acontributes to perturbation masses 6 without affecting characteristicsof the elastic acoustic wave. Therefore, it is possible to realize anangular rate sensor having high sensitivity.

FIG. 14 shows a flow chart of steps for manufacturing the angular ratesensor 1 includes a preparing step (S300) for preparing thesemiconductor substrate 2, a first electrode forming step (S302) fordepositing the first electrode 3 on the semiconductor substrate 2 atleast over a region above which perturbation masses 6 is formed, apiezoelectric film forming step (S304) for depositing the piezoelectricfilm 5 on at least one of the first electrode 3 and the semiconductorsubstrate 2, a contact holes forming step (S306) for forming the contactholes 5 a in the piezoelectric film 5, a second electrode forming step(S308) for depositing the second electrode 30 on the upper surface ofthe piezoelectric film 5, and a fabricating step (S310) for fabricatinga plurality of features on the piezoelectric film 5, wherein theplurality of features includes the perturbation masses 6, a drivingelectrode 7-10 for causing an elastic acoustic wave in the piezoelectricfilm 5, reflectors 11, 12 for reflecting the elastic acoustic wave so asto form a first and a second standing wave, and detecting electrodes13-16 for detecting a second standing wave caused by the Coriolis forceacting on the first standing wave with production technology forvery-large-scale integrated circuits (VLSIs).

FIG. 15 shows illustrates the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as in the method for manufacturing the angular rate sensoraccording to the first embodiment, a 400 μm or the more thick(100)-oriented silicon substrate is prepared (S300 in FIG. 14).

Next, as shown in FIG. 15A, a thin silicon dioxide film which is thinnerthan 1 μm is grown on the silicon substrate in the same way shown inFIG. 4A.

As shown in FIG. 15B, a several hundreds nanometer thick film ofimpurity-doped polycrystalline silicon is deposited in the same wayshown in FIG. 4B (S302 in FIG. 14). After this step, a formation of thefirst electrode 4 is finished.

In the next step, a several micrometer thick film of a piezoelectricfilm such as aluminum nitride (AlN), zinc oxide (ZnO), zirconate titante(PZT), lead titanate (PT), lithium tantalite (LiTaO₃), and lithiumtantalite (LT), is disposed so as to cover the whole region of an uppersurface of the first electrode 4 (S304 in FIG. 14). If an aluminumnitride (AlN) film is used as the first electrode 4, the AlN film ispatterned by a tetra methyl ammonium hydroxide-based solvent.

Then, as shown in FIG. 15C, contact holes 5 a are formed having aplurality of the apertures through the piezoelectric film 5 (S306 inFIG. 14). The piezoelectric film 5 is soaked in photoresist anddeveloped until the sections which have been exposed to ultra-violet(UV) light and are therefore soluble, are etched away.

Next, a thin film of metal or conductive metallic alloy includingaluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy, isdeposited using an e-beam evaporator, and patterned by a phosphoric(H₃PO₄)-based acid to form the second electrode 30 and the contact holes5 a. Thus, the second electrode 30 and the contact holes 5 a are formed(S308 in FIG. 14).

Next, as shown in FIG. 12D, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the perturbationmasses 6, the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 on an upper surface of the piezoelectric film5 (S310 in FIG. 14), in the same way shown in FIG. 4C.

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Third Embodiment

In the third embodiment described above in which the second electrode30, the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16 are disposed directly on the upper surface ofthe piezoelectric film 5.

As shown in FIG. 16, if necessary, a thin insulator film 20 is formedand inserted between the upper surface of the piezoelectric film 5.Consequently, the second electrode 30, the driving electrodes 7-10, thereflectors 11, 12, and the detecting electrodes 13-16 are formed on thethin insulator film 20. In this configuration, the second electrode 30,the driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are disposed on the thin insulator film 20. In thisconfiguration, the contact holes 5 a penetrate the thin insulator film20 so as to electrically connect the first electrode 4 to the secondelectrode 30.

Forth Embodiment

Referring to FIGS. 17A-19D, an angular rate sensor according to a forthembodiment of the present invention will now be explained. In the forthembodiment, the identical components in structure to those in the firstembodiment are assigned to the same reference numerals for simplicityand the description of these components are omitted from the detailedexplanations.

In the angular rate sensor according to the forth embodiment, there isprovided an angular rate sensor 1 having openings for accommodatingperturbation masses 6 in the piezoelectric film 5 so as to resolve aquestion how a polarization occurred along the z-direction in thepiezoelectric film 5 or surface charges generated at the upper and lowersurfaces of the piezoelectric film 5 should be removed.

FIG. 17A and FIG. 17B show an angular rate sensor 1 according to theforth embodiment. FIG. 17A shows a bird's-eye view of the angular ratesensor 1 and FIG. 17B shows a cross-sectional view taken along C-C linein FIG. 17A.

As shown in FIG. 17A, in the angular rate sensor 1 has the openings overthe predetermined region 60 for accommodating the perturbation masses 6,the piezoelectric material composing the piezoelectric film 5 isexpelled from the openings.

Even though the angular sensor 1 has the opening over the predeterminedregion 60 such that the piezoelectric film 5 is not formed over thepredetermined region 60 on which the perturbation masses 6 are disposed,a standing wave generated and rectified by the driving electrodes 7-10and the reflectors 11, 12, exist within the predetermined region 60 suchthat the Coriolis force acting on the perturbation masses 6 generate asecond elastic acoustic wave in the y-direction and then is detected bythe detecting electrodes 13-16 being alignment along the y-direction.

The angular rate sensor according to the forth embodiment, includes thesemiconductor substrate 2 such as silicon substrate, the insulator film3 such as silicon dioxide film disposed on the semiconductor substrate2, the first electrode 4 disposed on the first insulator film 3, thepiezoelectric film 5 disposed on the first electrode 4 in which theopening is formed by expelling the piezoelectric film 5 from thepredetermined region 60, the second insulator film 20 disposed on thepiezoelectric film 5 or the first electrode 4, the perturbation masses 6disposed on the predetermined region 60, the driving electrode 7-10 forcausing a first elastic acoustic wave in the piezoelectric film 5, thereflectors 11, 12 for reflecting the elastic acoustic wave caused by thedriving electrodes 7-10 to form a standing wave of the elastic acousticwave in the piezoelectric film 5, the detecting electrodes 13-16 fordetecting a second elastic wave generated by the Coriolis force actingon the perturbation masses 6, wherein the driving electrodes 7-10, thereflectors 11, 12, the detecting electrodes 13-16 are formed on thesecond insulator film 20.

In this configuration of the angular rate sensor 1, within thepredetermined region 60, the perturbation masses 6 is not formed on thepiezoelectric film 5 but on the thin insulator film 4. Thus, even if thepolarization between the upper and the lower surfaces of thepiezoelectric film 5 is generated since particles in the piezoelectricfilm 5 which are associated with the standing wave are displaced fromcorresponding neutral positions, the polarization in the piezoelectricfilm does not contribute to the vibration of the perturbation masses 6in the z-direction. As a result, a limitation of the displacement of thevibrating perturbation masses 6 accommodated into the openings in thez-direction is eliminated.

In this case the standing wave is excited in the first electrode 4 andthe thin insulator film 3, and at the upper surface of the semiconductorsubstrate 2. Thus, the perturbation masses 6 at the anti-nodes of thestanding wave are subject to an oscillation used as a referencevibration of the angular rate sensor. In consequence, it is possible tomeasure an angular rate in the same operating principle with thatutilized in the previous embodiments.

A method for fabricating the angular rate sensor 1 according to theforth embodiment will be described.

FIG. 18 is a flow chart of steps for manufacturing the angular ratesensor 1 includes a preparing step (S400) for preparing thesemiconductor substrate 2, a first electrode forming step (S402) fordepositing the first electrode 3 on the semiconductor substrate 2 atleast over a square region above which perturbation masses 6 is formed,a piezoelectric film forming step (S404) for depositing thepiezoelectric film 5 on at least one of the first electrode 3 and thesemiconductor substrate 2, an opening forming step (S406) for formingthe opening in the piezoelectric film 5 over the predetermined region 60above which a perturbation masses 6 are disposed, an insulator thin filmforming step (S408) for forming the insulator thin film on thepiezoelectric film 5 and an exposed surface of the opening, and afabricating step (S410) for fabricating a plurality of features on thepiezoelectric film 5, wherein the plurality of features includes theperturbation masses 6, a driving electrode 7-10 for causing an elasticacoustic wave in the piezoelectric film 5, reflectors 11, 12 forreflecting the elastic acoustic wave so as to form a first and a secondstanding wave, and detecting electrodes 13-16 for detecting a secondstanding wave caused by the Coriolis force acting on the first standingwave with production technology for very-large-scale integrated circuits(VLSIs).

FIGS. 19A to 19D illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as in the method for manufacturing the angular rate sensor 1according to the first embodiment, a (100)-oriented silicon substrate of400 μm thickness is prepared (S400 in FIG. 18).

Next, as shown in FIG. 19A, a thin silicon dioxide film which is thinnerthan 1 μm, is grown on the silicon substrate in the same way shown inFIG. 4A.

As shown in FIG. 19B, a thick film which is of order of several hundredsnanometer of impurity-doped polycrystalline silicon is deposited in thesame way shown in FIG. 4B. After this step, a formation of a firstelectrode 4 is finished (S402 in FIG. 18).

In the next step, a several micrometer thick film of a piezoelectricfilm such as aluminum nitride (AlN), zinc oxide (ZnO), zirconate titante(PZT), lead titanate (PT), lithium tantalite (LiTaO₃), and lithiumtantalite (LT), is disposed so as to cover the whole region of an uppersurface of the first electrode 4 (S404 in FIG. 18). If an aluminumnitride (AlN) film is used as the first electrode 4, the AlN film ispatterned by a tetra methyl ammonium hydroxide-based solvent.

Then, as shown in FIG. 19C, the opening is formed within thepredetermined region 60 through the piezoelectric film 5 (S406 in FIG.18). The piezoelectric film 5 is soaked in photoresist and developeduntil the sections which have been exposed to ultra-violet (UV) lightand are therefore soluble, are etched away.

Next, a thin silicon dioxide film 20 is formed on the piezoelectric film5 or the first electrode 4 within the predetermined region 60 on whichthe opening is formed (S408 in FIG. 18).

Next, as shown in FIG. 19D, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the secondelectrode 30 and perturbation masses 6 (S410 in FIG. 18).

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of First to Fourth Embodiments

In the first to forth embodiments, the first electrode 4 is formed overthe whole upper surface of the thin insulator film covering the surfaceof the semiconductor substrate 2. That is, the first electrode 4 isdisposed below the driving electrodes 7-10. In the configurationsaccording to the first to third embodiments, the perturbation masses 6vibrates in the z-direction with large amplitude since the drivingelectrode 7-10 generates the elastic acoustic wave in the piezoelectricfilm 5 over a wide region including not only the predetermined region 60on which the perturbation masses 6 are disposed but also an outer regionof the predetermined region 60 on which the driving electrodes 7-10, thereflectors 11, 12 are disposed. However, the first electrode 4 can beformed only below the predetermined region 60. In this configuration,the same effects obtained by the angular rate sensors according to thefirst to third embodiments are attained.

Further, in the angular rate sensors according to the first to forthembodiments, the driving IDTs and detecting IDTs consists of thecomb-shaped electrode 7-10, 13-16. However, the driving IDTs anddetecting IDTs are limited to these configurations but other shapedelectrodes can be used for the components of the driving IDTs and thedetecting IDTs.

Further, in the angular rate sensors according to the first to thirdembodiments, the driving electrodes 7-10 are located such that thedriving electrodes 7-8 are disposed at one side of the predeterminedregion 60 and the driving electrodes 9-10 are disposed at another sideof the predetermined region 60. However, it is not necessary to form thedriving electrodes 7-10 at both sides of the predetermined region 60. Ina modification of the angular rate sensor according to the first tothird embodiments, at least one pairs of the driving electrodes 7-8 andthe driving electrodes 9-10 is formed.

Still further, in the modification of the angular rate sensor accordingto the forth embodiment, the opening is filled with some elasticmaterial including metal, metallic alloy, semiconductor, and insulatorwith the exception of piezoelectric material. The perturbation masses 6are formed on the above mentioned elastic material. In thisconfiguration, even if elastic acoustic waves are caused within thepredetermined region 60, electric polarization is not generated belowthe predetermined region 60. Therefore, a high sensitive angular ratesensor is achieved.

Fifth Embodiment

Referring to FIGS. 20A-22D, an angular rate sensor according to thefifth embodiment of the present invention will now be explained. In thisembodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

FIG. 20A and FIG. 20B show an angular rate sensor 1 according to thefifth embodiment. FIG. 20A shows a bird's-eye view of the angular ratesensor 1 and FIG. 20B shows a cross-sectional view taken along A-A linein FIG. 20A.

The angular rate sensor 1 according to the fifth embodiment hasperturbation masses 6 whose mass density is larger than that of theprevious embodiments. In, more detail, a material of which theperturbation masses 6 is made has higher mass density than that ofanother material of which the driving electrodes 7-10, the reflectors11, 12, and the detecting electrodes 13-16. If the driving electrodes7-10, the reflectors 11, 12, and the detecting electrodes 13-16 are madeof phosphor-doped polycrystalline silicon, it is suitable that theperturbation mass is made of one of platinum (Pt), gold (Au), andtungsten (W). Platinum (Pt), gold (Au), and tungsten (W) have massdensity of 21.4 g/cm³, 19.3 g/cm³, and 19.1 g/cm³, respectively. Furtherit is preferable that thickness of the perturbation masses 6 is largerthan twice of that of the driving electrodes 7-10.

In such a configuration of the perturbation masses 6, the Coriolis forceacting on the perturbation masses 6 is emphasized so as to increase thedisplacement of the perturbation masses 6 in the z-direction. Inconsequence, it is possible to realize an angular rate sensor 1 havinghigh sensitivity. Further the angular rate sensor having highsensitivity can be miniaturized with production technology forvery-large-scale integrated circuits (VLSIs).

In more detail, the perturbation masses 6 are usually made of aluminum(Al) whose mass density is 2.69 g/cm³ that is also widely used forwiring. However the mass density of aluminum (Al) is not enough togenerate the large Coriolis force that is proportional to both velocityand mass density. If the perturbation masses 6 are made of one of one ofplatinum (Pt), gold (Au), and tungsten (W), whose mass densities havefive times larger than that of aluminum (Al) and the thickness of theperturbation masses 6 has twice larger than that of the drivingelectrodes 7-10, the sensitivity of the angular rate sensor 1 isimproved by single-digit.

In the above description, the perturbation masses 6, the drivingelectrodes 7-10, reflectors 11, 12, and the detecting electrodes 13-16are made of metal or metal alloys. However, other materials whose massdensity is larger are applicable.

A method for fabricating the angular rate sensor 1 according to thefifth embodiment will be described with reference to FIGS. 16 and 17.

FIG. 21 shows a flow chart of steps for manufacturing the angular ratesensor 1 includes a preparing step (S500) for preparing thesemiconductor substrate 2, a first electrode forming step (S502) fordepositing the first electrode 3 on the semiconductor substrate 2 atleast over a square region above which perturbation masses 6 is formed,a piezoelectric film forming step (S504) for depositing thepiezoelectric film 5 on at least one of the first electrode 3 and thesemiconductor substrate 2, a first fabricating step (S506) forfabricating a plurality of features on the piezoelectric film 5 using afirst material such as a metal and a metal alloy, wherein the pluralityof features includes the driving electrode 7-10 for causing an elasticacoustic wave in the piezoelectric film 5, reflectors 11, 12 forreflecting the elastic acoustic wave so as to form a first and a secondstanding wave, and detecting electrodes 13-16 for detecting a secondstanding wave caused by the Coriolis force acting on the first standingwave, a second fabricating step (S508) for fabricating the perturbationmasses 6 using a second material whose mass density has larger than thatof the first material. In all steps mentioned above, productiontechnology for very-large-scale integrated circuits (VLSIs) isapplicable.

FIGS. 22A to 22D illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as in the method for manufacturing the angular rate sensor 1according to the first embodiment, a (100)-oriented silicon substrate of400 μm thickness is prepared (S500 in FIG. 21).

Next, as shown in FIG. 22A, a thin silicon dioxide film which is thinnerthan 1 μm, is grown on the silicon substrate in the same way shown inFIG. 4A.

As shown in FIG. 22B, a thick film which is of order of several hundredsnanometer of impurity-doped polycrystalline silicon is deposited in thesame way shown in FIG. 4B. In other words, this step is a firstelectrode forming step for forming the first electrode 4 on the thinsilicon dioxide (S502 in FIG. 21).

In the next step, a several micrometer thick film of a piezoelectricfilm such as aluminum nitride (AlN), zinc oxide (ZnO), zirconate titante(PZT), lead titanate (PT), lithium tantalite (LiTaO₃), and lithiumtantalite (LT), is disposed so as to cover the whole region of an uppersurface of the first electrode 4 in order to form a piezoelectric film 5(S504 in FIG. 21). If an aluminum nitride (AlN) film is used as thefirst electrode 4, the AlN film is patterned by a tetra methyl ammoniumhydroxide-based solvent.

Next, as shown in FIG. 12C, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the drivingelectrodes 7-10, reflectors 11, 12, and the detecting electrodes 13-16on an upper surface of the piezoelectric film 5 (S506 in FIG. 21), inthe same way shown in FIG. 4C.

Next, as shown in FIG. 22D, a thin film of metal or conductive metallicalloy including platinum (Pt), gold (Au), and tungsten (W) is depositedusing an e-beam evaporator, and patterned by a phosphoric (H₃PO₄)-basedacid to form the perturbation masses 6 (S508 in FIG. 21).

In this step, it is preferable that a selective etching technique havingthe proper etching ratio of one material of the perturbation masses 6 toanother material of the driving electrodes 7-10, reflectors 11, 12, andthe detecting electrodes 13-16 is used.

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Fifth Embodiment

In the fifth embodiment described above in which the perturbationmasses6, the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16 are disposed directly on the upper surface ofthe piezoelectric film 5.

As shown in FIG. 23, if necessary, an thin insulator film 20 is formedand inserted between the upper surface of the piezoelectric film 5.Consequently, the second electrode 30, the driving electrodes 7-10, thereflectors 11, 12, and the detecting electrodes 13-16 are formed on thethin insulator film 20. In this configuration, the second electrode 30,the driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are disposed on the thin insulator film 20. In thisconfiguration, it is possible to prevent etching the piezoelectric film5 while the thin film of metal or conductive metallic alloy is patternedto form the second electrode 30, the perturbation masses 6, and thedriving electrodes 7-10.

Further, even though the angular rate sensor according to the fifthembodiment in which the first electrode 4 is formed below the drivingelectrodes 7-10 and perturbation masses 6 is made of a metal or a metalalloy whose mass density is larger than that of driving electrodes 7-10,is disclosed, the first electrode 4 is not a crucial component for anangular rate sensor 1 having higher sensitivity if perturbation masses 6of the angular rate sensor 1 is made of a metal or a metal alloy whosemass density is larger than that of driving electrodes 7-10.

Sixth Embodiment

Referring to FIGS. 24-28B, an angular rate sensor according to the sixthembodiment of the present invention will now be explained. In thisembodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

The angular rate sensor according to the sixth embodiment hasperturbation masses 6 which have different shape from that of the firstembodiment, although the angular rate sensor 1 according to thisembodiment includes the semiconductor substrate 2 such as siliconsubstrate, the insulator film 3 such as silicon dioxide film disposed onthe semiconductor substrate 2, the first electrode 4 disposed on thefirst insulator film 3, the piezoelectric film 5 disposed on the firstelectrode 4, the perturbation masses 6 which emphasis the Coriolis forceduring the angular rate sensor rotating, the driving electrode 7-10 forcausing a first elastic acoustic wave in the piezoelectric film 5, thereflectors 11, 12 for reflecting the elastic acoustic wave caused by thedriving electrodes 7-10 to form a standing wave of the elastic acousticwave in the piezoelectric film 5, the detecting electrodes 13-16 fordetecting a second elastic wave generated by the Coriolis force actingon the perturbation masses 6, wherein the driving electrodes 7-10, thereflectors 11, 12, the detecting electrodes 13-16 are formed on thepiezoelectric film 5. The angular rate sensor according to thisembodiment has a plurality of needle-shaped perturbation masses 6.

FIG. 24 shows a cross-sectional view of the angular rate sensoraccording to the sixth embodiment of the present invention. As shown inFIG. 24, the angular rate sensor according to this embodiment hasperturbation masses 6 which are longer than those of the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16 so as to obtain larger amplitude of the Coriolis force due toheavy mass of each perturbation mass 6. In FIG. 24, it is clarified thefact that the angular rate sensor according to this embodiment has theplurality of the needle-shaped perturbation masses 6.

The needle-shaped perturbation masses 6 contribute to high sensitivityof the angular rate sensor 1 since the Coriolis force is written in aform

=2m

×

, where the velocity of each perturbation mass 6,

, each perturbation mass 6 has a mass, m, and the angular rotation

.

In this configuration of the perturbation masses 6 which is longer thanthose of the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16, the magnitude of the Coriolis force isincreased so that the propagating wave generated by the Coriolis forcein the y-direction has larger amplitude.

In consequence, it is possible to realize an angular rate sensor havinghigh sensitivity.

A method for fabricating the angular rate sensor 1 with productiontechnology for very-large-scale integrated circuits (VLSIs) will bedescribed.

FIG. 25 shows a flow chart of steps for manufacturing the angular ratesensor 1 according to the sixth embodiment includes a preparing step(S600) for preparing the semiconductor substrate 2, a first electrodeforming step (S602) for depositing the first electrode 4 on thesemiconductor substrate 2 at least over a square region above whichperturbation masses 6 is formed, a piezoelectric film forming step(S604) for depositing the piezoelectric film 5 on at least one of thefirst electrode 4 and the semiconductor substrate 2, a first fabricatingstep (S606) for fabricating a plurality of features on the piezoelectricfilm 5 using a first material such as a metal and a metal alloy, whereinthe plurality of features includes the driving electrode 7-10 forcausing an elastic acoustic wave in the piezoelectric film 5, reflectors11, 12 for reflecting the elastic acoustic wave so as to form a firstand a second standing wave, and detecting electrodes 13-16 for detectinga second standing wave caused by the Coriolis force acting on the firststanding wave, a second fabricating step (S608) for fabricating theperturbation masses 6 which are longer than those of the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16. In all steps mentioned above, production technology forvery-large-scale integrated circuits (VLSIs) is applicable.

FIGS. 26A to 26E illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as in the method for manufacturing the angular rate sensor 1according to the first embodiment, a (100)-oriented silicon substrate of400 μm thickness is prepared (S600 in FIG. 25).

Next, as shown in FIG. 26A, a thin silicon dioxide film 3 which isthinner than 1 μm, is grown on the silicon substrate in the same wayshown in FIG. 4A.

As shown in FIG. 26B, a thick film 4 which is of order of severalhundreds nanometer of impurity-doped polycrystalline silicon isdeposited in the same way shown in FIG. 4B. In this step, a firstelectrode 4 is formed (S602 in FIG. 25). In the next step, a severalmicrometer thick film of a piezoelectric film such as aluminum nitride(AlN), zinc oxide (ZnO), zirconate titante (PZT), lead titanate (PT),lithium tantalite (LiTaO₃), and lithium tantalite (LT), is disposed soas to cover the whole region of an upper surface of the first electrode4 (S604 in FIG. 25). If an aluminum nitride (AlN) film is used as thefirst electrode 4, the AlN film is patterned by a tetra methyl ammoniumhydroxide-based solvent.

Next, as shown in FIG. 26C, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the drivingelectrodes 7-10, reflectors 11, 12, and the detecting electrodes 13-16on an upper surface of the piezoelectric film 5 (S606 in FIG. 25), inthe same way shown in FIG. 4C.

Next, as shown in FIG. 26D, a resist thick film 70 of 10 μm and morethickness which is to be masked is formed on the driving electrodes7-10, reflectors 11, 12, and the detecting electrodes 13-16 which coverson the surface of the piezoelectric film 5, or on the surface of thepiezoelectric film 5, and apertures in the shape of a pattern of whichthe perturbation masses 6 are arranged. Next, metal or conductivemetallic alloy is deposited to form the needle-shaped perturbationmasses 6 into the apertures of the resist thick film 70. Thus, theplurality of the needle-shaped perturbation masses 6 having high aspectratio is formed. The aspect ratio of a two-dimensional shape is definedas the ratio of its longer dimension to its shorter dimension.Therefore, an aspect ratio of the perturbation masses 6 is defined asthe ratio of the height to the width of the bottom thereof. Instead of adeposition method for fabricating the perturbation masses 6 mentionedabove, electro plating and electroless plating is also applicable toform the perturbation masses 6. In the case of the electro plating usinga shield film, the shield film is deposited and a resist thick film isformed. Then, the perturbation masses 6 are pattered.

Next, as shown in FIG. 20E, the resist thick film 70 and the film ofmetal or conductive metallic alloy formed on the resist thick film 70are removed. So that the plurality of the needle-shaped perturbationmasses 6 are formed (S608 in FIG. 25).

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Sixth Embodiment

In the angular rate sensor 1 according to the sixth embodiment, eachperturbation mass 6 has high aspect ratio such as the needle-shaped.

In a modification of the angular rate sensor according to the sixthembodiment, the perturbation masses 6 are formed such that on the topsof all needle-shaped perturbation masses 6 a single plate is placed. Ifthe needle-shaped perturbation masses 6 are considered as pillars of ahouse, the single plate can be considered as a roof.

This configuration of the perturbation masses 6 is realized by disposingmetal or conductive metallic alloy to form the needle-shapedperturbation masses 6 into the opening of the resist thick film 70 andthe roof of the perturbation masses 6. The roof of the perturbationmasses 6 improves the stiffness thereof such that a resonant frequencyof the standing wave can be increase to be higher. This leads to anexpectation that the angular rate sensor is insensitive to externalvibration noise.

In a method for fabricating the perturbation masses 6 having the roof, aresist thick film 70 of 10 μm and more thickness which is to be maskedis formed on the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 which covers on the surface of thepiezoelectric film 5, or on the surface of the piezoelectric film 5, andapertures in the shape of a pattern of which the perturbation masses 6and an opening in the shape of the roof are arranged. Next, metal orconductive metallic alloy is deposited to form the needle-shapedperturbation masses 6 with the roof into the apertures and the openingof the resist thick film 70. The shape of the roof is not only platy butalso lattice-like. However, the former is preferable in the point ofview of stiffness.

In the sixth embodiment described above in which the second electrode30, the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16 are disposed directly on the upper surface ofthe piezoelectric film 5, as shown in FIGS. 19 and 21.

As shown in FIGS. 28A and 28B, if necessary, a thin insulator film 20 isformed and inserted between the upper surface of the piezoelectric film5 and the features which include the perturbation masses 6, the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16. Consequently, the second electrode 30, the driving electrodes7-10, the reflectors 11, 12, and the detecting electrodes 13-16 areformed on the thin insulator film 20. In this configuration, the secondelectrode 30, the driving electrodes 7-10, the reflectors 11, 12, andthe detecting electrodes 13-16 are disposed on the thin insulator film20. In this configuration, it is possible to prevent etching thepiezoelectric film 5 while the thin film of metal or conductive metallicalloy is patterned to form the second electrode 30, the perturbationmasses 6, and the driving electrodes 7-10.

Further, even though the angular rate sensor according to the sixthembodiment in which the first electrode 4 is formed below the drivingelectrodes 7-10 and perturbation masses 6 is made of a metal or a metalalloy whose mass density is larger than that of driving electrodes 7-10,is disclosed, the first electrode 4 is not a crucial component for anangular rate sensor 1 having higher sensitivity if perturbation masses 6of the angular rate sensor 1 is made of a metal or a metal alloy whosemass density is larger than that of driving electrodes 7-10.

Seventh Embodiment

Referring to FIGS. 29A-32, an angular rate sensor according to theseventh embodiment of the present invention will now be explained. Inthis embodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

FIG. 29A and FIG. 29B show an angular rate sensor 1 according to theseventh embodiment. FIG. 29A shows a bird's-eye view of the angular ratesensor 1 and FIG. 29B shows a cross-sectional view taken along B-B linein FIG. 29A.

As shown in FIG. 29A, an angular rate sensor 1 in this embodiment havinga plurality of trenches 2 a in the semiconductor substrate 2 into whichthe perturbation masses 6 are accommodated. In this configuration, theperturbation masses 6 are located under the piezoelectric film 5. Moreconcretely, the plurality of the trenches 2 a whose number is the samewith that of the perturbation masses 6 is formed in the semiconductorsubstrate 2. Each of the perturbation masses 6 is accommodated into thecorresponding trench 2 a. Then, the insulator film 3 such as silicondioxide film is formed so as to cover the perturbation masses 6. On theinsulator film 3, the first electrode 4 disposed on the first insulatorfilm 3, the piezoelectric film 5 disposed on the first electrode 4, theperturbation masses 6 which emphasis the Coriolis force during theangular rate sensor rotating, the driving electrode 7-10 for causing afirst elastic acoustic wave in the piezoelectric film 5, the reflectors11, 12 for reflecting the elastic acoustic wave caused by the drivingelectrodes 7-10 to form a standing wave of the elastic acoustic wave inthe piezoelectric film 5, the detecting electrodes 13-16 for detecting asecond elastic wave generated by the Coriolis force acting on theperturbation masses 6, are formed. In this configuration, theperturbation masses 6 are made of a material such as platinum (Pt), gold(Au), and tungsten (W) which have higher mass density than that of thesemiconductor substrate 3.

As shown in FIG. 29B, the angular rate sensor 1 having the perturbationmasses 6 embedded into the semiconductor substrate 3 and made of amaterial having higher mass density than that of the semiconductorsubstrate 3, achieves higher sensitivity since the heavy perturbationmasses 6 is obtained. As a result, a limitation of the displacement ofthe vibrating perturbation masses 6 accommodated into the openings inthe z-direction is eliminated.

A method for fabricating the angular rate sensor 1 according to theseventh embodiment will be described.

FIG. 30 shows a flow chart of steps for manufacturing the angular ratesensor 1 according to the forth embodiment includes a preparing step(S700) for preparing the semiconductor substrate 2, a trench formingstep (S702) for forming a plurality of trenches 2 a in the semiconductorsubstrate 2, a first insulator film forming step (S704) for disposingthe insulator film 3 on an exposed surface of every trench 2 a, aperturbation mass forming step (S706) for forming the perturbationmasses 6 in the trenches 2 a whose surfaces are coated by the insulatorfilm 3, a second insulator film forming step (S708) for disposing theinsulator film 3 on the top of the trenches 2 a and the insulator film 3formed in the first insulator film forming step, a piezoelectric filmforming step (S710) for forming the piezoelectric film 5 so as to coverthe plurality of the trenches, the perturbation masses and thesemiconductor substrate, and a fabricating step (S712) for fabricating aplurality of features on the piezoelectric film 5, wherein the pluralityof features includes the perturbation masses 6, a driving electrode 7-10for causing an elastic acoustic wave in the piezoelectric film 5,reflectors 11, 12 for reflecting the elastic acoustic wave so as to forma first and a second standing wave, and detecting electrodes 13-16 fordetecting a second standing wave caused by the Coriolis force acting onthe first standing wave.

FIGS. 31A-31G illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as shown in FIG. 31A in the method for manufacturing the angularrate sensor 1 according to the first embodiment, a (100)-orientedsilicon substrate 2 of 400 μm thickness is prepared (S700 in FIG. 30).

Next, the trenches 2 a are formed in the semiconductor substrate 2 suchas silicon substrate using photolithography technique (S702 in FIG. 30).For example, if silicon substrate is used as the semiconductorsubstrate, an upper surface of the silicon substrate is cleaned by usingacetone, isopropanol and trichloroethylene, in turn. The surface of thesilicon substrate is then thoroughly rinsed in de-ionized water andsubsequently heated on a hot plate to remove surface moisture. Uponcooling the substrate on a heat sinking plate, photoresist is then spincoated on the silicon substrate after soaking the hexamethyldisilazane,an adhesion agent. Then the soft-bake process is performed in which thesubstrate is heated. A negative-mask is set in place over thephotoresist. The negative mask is a template defining the patterns ofthe substrate. The silicon substrate is then exposed to ultra-violet(UV) light such that the regions of the resist which are exposed becomesoluble to the developer. Thus, a pattern is formed having a pluralityof apertures therethrough. The substrate is soaked in photoresist anddeveloped until the sections which has been exposed to UV light, and aretherefore soluble, are etched away.

For example, dry etching is applicable to etch the trenches 2 a. A gasincluding fluorine, such as a CF-type gas including C4F8, or a SF-typegas including SF6, is used as etching gas for the dry etching. Theetching gas is changed into plasma to produce fluorine radicals, andetching is performed by processing the silicon substrate with thefluorine radicals.

Next, as shown in FIG. 31B, a thin silicon dioxide film which is thinnerthan 1 μm, is thermally grown on the exposed surface of the trenches 2 aand upper surface of the silicon substrate 2 (S704 in FIG. 30).

Next, as shown in FIG. 31C, a metal or conductive metallic alloy 40including platinum (Pt), gold (Au), and tungsten (W) is deposited usingan e-beam evaporator, and patterned by a phosphoric (H₃PO₄)-based acidon the exposed surface of the trenches 2 a and upper surface of thesilicon substrate 2 to form the perturbation masses 6 (S706 in FIG. 30).

Next, as shown in FIG. 31D, the upper surface of the metal or conductivemetallic alloy 40 is etched back until the surface of the silicondioxide 3 is exposed (S708 in FIG. 30). In this step, not only dryetching technique but also chemical mechanical polishing (CMP) techniqueare applicable.

Next, as shown in FIG. 31E, a silicon dioxide film is thermally grown soas to cover the trenches 2 a and from the perturbation masses 6 insidethe silicon dioxide. 3

Next, as shown in FIG. 31F, a several micrometer thick film of apiezoelectric film such as aluminum nitride (AlN), zinc oxide (ZnO),zirconate titante (PZT), lead titanate (PT), lithium tantalite (LiTaO₃),and lithium tantalite (LT), is disposed so as to cover the whole regionof an upper surface of the first electrode 4 (S710 in FIG. 30). If analuminum nitride (AlN) film is used as the first electrode 4, the AlNfilm is patterned by a tetra methyl ammonium hydroxide-based solvent.

Next, as shown in FIG. 31G, a thin film of metal or conductive metallicalloy including aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the drivingelectrodes 7-10, reflectors 11, 12, and the detecting electrodes 13-16on an upper surface of the piezoelectric film 5, in the same way shownin FIG. 4C(S712 in FIG. 30).

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Seventh Embodiment

In the seventh embodiment described above in which the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16 are disposed directly on the upper surface of the piezoelectricfilm 5.

As shown in FIG. 32, if necessary, a thin insulator film 20 is formedand inserted between the upper surface of the piezoelectric film 5.Consequently, the second electrode 30, the driving electrodes 7-10, thereflectors 11, 12, and the detecting electrodes 13-16 are formed on thethin insulator film 20. In this configuration, the second electrode 30,the driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are disposed on the thin insulator film 20. In thisconfiguration, it is possible to prevent etching the piezoelectric film5 while the thin film of metal or conductive metallic alloy is patternedto form the second electrode 30, the perturbation masses 6, and thedriving electrodes 7-10.

Further, even though the angular rate sensor according to the seventhembodiment in which the first electrode 4 is formed below the drivingelectrodes 7-10 and perturbation masses 6 is made of a metal or a metalalloy whose mass density is larger than that of driving electrodes 7-10,is disclosed, the first electrode 4 is not a crucial component for anangular rate sensor 1 having higher sensitivity if perturbation masses 6of the angular rate sensor 1 is made of a metal or a metal alloy whosemass density is larger than that of driving electrodes 7-10.

Eighth Embodiment

Referring to FIGS. 33A-38, an angular rate sensor according to theeighth embodiment of the present invention will now be explained. Inthis embodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

FIG. 33A and FIG. 33B show an angular rate sensor 1 according to theeighth embodiment. FIG. 33A shows a bird's-eye view of the angular ratesensor 1 and FIG. 33B shows a cross-sectional view taken along C-C linein FIG. 33A.

As shown in FIG. 33A, an angular rate sensor in this embodiment isarranged such that all of the driving electrodes 7-10, the reflectors11, 12, and the detecting electrodes 13-16 are immersed into thesemiconductor substrate 2 such as silicon substrate. Therefore, all ofthe driving electrodes 7-10, the reflectors 11, 12, and the detectingelectrodes 13-16 are located under the piezoelectric film 5. An thininsulator film 20 is formed on the piezoelectric film 5. In thisarrangement, the first electrode 4 is not needed for the neutralizingthe polarization occurred in the z-direction of the piezoelectric film5. In more detail, the angular rate sensor 1 has a plurality of trenches2 a in the semiconductor substrate 2 into which not only theperturbation masses 6 but also the driving electrodes 7-10, thereflectors 11, 12, and the detecting electrodes 13-16 are accommodated.The surfaces of the trenches 2 a is coated by the thin insulator film 4such as silicon dioxide film. The perturbation masses 6, the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16 are formed on the thin insulator film 4 such that all of theperturbation masses 6, the driving electrodes 7-10, the reflectors 11,12, and the detecting electrodes 13-16 are accommodated into thecorresponding trench 2 a. After the perturbation masses 6, the drivingelectrodes 7-10, the reflectors 11, 12, and the detecting electrodes13-16 are disposed, the insulator film 3 is additionally grown so as toembed features 6-16 thereinto. On the insulator film 4, thepiezoelectric film 5 and a further thin insulator film 20 are formed, inthis order.

As shown in FIG. 33B, the angular rate sensor having the perturbationmasses 6, the driving electrodes 7-10, the reflectors 11, 12, and thedetecting electrodes 13-16, all embedded into the semiconductorsubstrate 3 and the perturbation masses 6 made of a material havinghigher mass density than that of the semiconductor substrate 3, achieveshigher sensitivity since the heavy perturbation masses 6 is obtained. Asa result, a limitation of the displacement of the vibrating perturbationmasses 6 accommodated into the openings in the z-direction iseliminated.

In the angular rate sensor of this type, the driving electrodes 7-10 andthe detecting electrodes 13-16 are electrically connected to theexternal driving and measuring circuit via contact holes (not shown infigures) formed in the further insulator film 20 and the piezoelectricfilm 5.

A method for fabricating the angular rate sensor 1 according to theeighth embodiment will be described.

FIG. 34 shows a flow chart of steps for manufacturing the angular ratesensor 1 according to the eighth embodiment includes a preparing step(S800) for preparing the semiconductor substrate 2, a trench formingstep (S802) for forming a plurality of trenches 2 a in the semiconductorsubstrate 2, a first insulator film forming step (S804) for disposingthe insulator film 3 on an exposed surface of every trench 2 a, afabricating step (S806) for fabricating a plurality of features in theinsulator film 3, wherein the plurality of features includes a drivingelectrode 7-10 for causing an elastic acoustic wave in the piezoelectricfilm 5, reflectors 11, 12 for reflecting the elastic acoustic wave so asto form a first and a second standing wave, and detecting electrodes13-16 for detecting a second standing wave caused by the Coriolis forceacting on the first standing wave, a perturbation mass forming step(S808) for forming the perturbation masses 6 in the trenches 2 a whosesurfaces are coated by the insulator film 3, a second insulator filmforming step (S810) for disposing the insulator film 3 on the top of thetrenches 2 a and the insulator film 3 formed in the first insulator filmforming step, and a piezoelectric film forming step (S812) for formingthe piezoelectric film 5 so as to cover the plurality of the trenches,the perturbation masses and the semiconductor substrate.

FIGS. 35A-35G illustrate the steps performed in the method formanufacturing the angular rate sensor 1 according to this embodiment.

First, as shown in FIG. 35A in the method for manufacturing the angularrate sensor 1 according to the first embodiment, a (100)-orientedsilicon substrate 2 of 400 μm thickness is prepared (S800 in FIG. 34).

Next, the trenches 2 a are formed for forming the driving electrodes7-10, the reflectors 11, 12, the detecting electrodes 13-16 in thesemiconductor substrate 2 such as silicon substrate usingphotolithography technique (S802 in FIG. 34). For example, if siliconsubstrate is used as the semiconductor substrate, an upper surface ofthe silicon substrate is cleaned by using acetone, isopropanol andtrichloroethylene, in turn. The surface of the silicon substrate is thenthoroughly rinsed in de-ionized water and subsequently heated on a hotplate to remove surface moisture. Upon cooling the substrate on a heatsinking plate, photoresist is then spin coated on the silicon substrateafter soaking the hexamethyldisilazane, an adhesion agent. Then thesoft-bake process is performed in which the substrate is heated. Anegative-mask is set in place over the photoresist. The negative mask isa template defining the patterns of the substrate. The silicon substrateis then exposed to ultra-violet (UV) light such that the regions of theresist which are exposed become soluble to the developer. Thus, apattern is formed having a plurality of apertures therethrough. Thesubstrate is soaked in photoresist and developed until the sectionswhich has been exposed to UV light, and are therefore soluble, areetched away to form the driving electrodes 7-10, the reflectors 11, 12,the detecting electrodes 13-16.

For example, dry etching is applicable to etch the trenches 2 a. A gasincluding fluorine, such as a CF-type gas including C4F8, or a SF-typegas including SF6, is used as etching gas for the dry etching. Theetching gas is changed into plasma to produce fluorine radicals, andetching is performed by processing the silicon substrate with thefluorine radicals.

Next, as shown in FIG. 35B, a thin silicon dioxide film which is thinnerthan 1 μm, is thermally grown on the exposed surface of the trenches 2 aand upper surface of the silicon substrate 2 (S804 in FIG. 34).

Next, as shown in FIG. 35C, a metal or conductive metallic alloy 50including platinum (Pt), gold (Au), and tungsten (W) is deposited usingan e-beam evaporator, and patterned by a phosphoric (H₃PO₄)-based acidon the exposed surface of the trenches 2 a and upper surface of thesilicon substrate 2 to form the driving electrodes 7-10, the reflectors11, 12, the detecting electrodes 13-16.

Next, as shown in FIG. 35D, the upper surface of the metal or conductivemetallic alloy 50 is etched back until the surface of the silicondioxide 3 is exposed. In this step, not only dry etching technique butalso chemical mechanical polishing (CMP) technique are applicable.

Next, as shown in FIG. 35E, a silicon dioxide film is thermally grown soas to cover the trenches 2 a and from the driving electrodes 7-10, thereflectors 11, 12, the detecting electrodes 13-16 inside the siliconsubstrate 2. In this step, the driving electrodes 7-10, the reflectors11, 12, the detecting electrodes 13-16 inside the silicon substrate 2are finished to be formed (S806 in FIG. 34).

Next, the trenches 2 a for forming the perturbation masses 6 are formedin the semiconductor substrate 2 such as silicon substrate usingphotolithography technique, in the similar method mentioned above. Ametal or conductive metallic alloy is deposited using an e-beamevaporator, and patterned by a phosphoric (H₃PO₄)-based acid on theexposed surface of the trenches 2 a and upper surface of the siliconsubstrate 2 to form the perturbation masses 6.

The perturbation masses 6 a is formed by growing silicon dioxide film onthe silicon dioxide 3 so as to cover the trenches 2 a and from theperturbation masses 6 (S808 in FIG. 34).

Next, as shown in FIG. 35G, a several micrometer thick film of apiezoelectric film such as aluminum nitride (AlN), zinc oxide (ZnO),zirconate titante (PZT), lead titanate (PT), lithium tantalite (LiTaO₃),and lithium tantalite (LT), is disposed so as to cover the whole regionof an upper surface of the first electrode 4 (5812 in FIG. 34). If analuminum nitride (AlN) film is used as the first electrode 4, the AlNfilm is patterned by a tetra methyl ammonium hydroxide-based solvent.

Next, a silicon dioxide film is thermally grown to form the thininsulator film 20 on the piezoelectric film 5.

Finally, the supplying driving voltage lines and ground lines are formedand connected to the driving electrodes 7-10, reflectors 11, 12, and thedetecting electrodes 13-16 by wire bonding.

Modification of Eighth Embodiment

As shown in FIG. 36, in order to fabricate the heavy perturbation masses6, it is preferable that the depth of the perturbation masses 6 islarge. In this modification of the eighth embodiment, each of theperturbation masses 6 are shaped like a needle.

Further, if the perturbation masses 6 is made of a metal or a metalalloy whose mass density is larger than that of driving electrodes 7-10,an angular rate sensor 1 having higher sensitivity is realized.

Further, as shown in FIG. 37, it is preferable that a conductive film 60is formed on the piezoelectric substrate 5. The angular rate sensorhaving the conductive film 60 formed on the piezoelectric film 5 becomesto be possible to generate and keep a predetermined voltage between thedriving IDT and the conductive film such that electric power isefficiently inputted into the piezoelectric film since the electricpotential level of the piezoelectric film 5 is capable to be keptconstant. Further, if the conductive film 60 is connected to a ground,the conductive film acts as a shield. This fact leads to an expectationthat the angular rate sensor 1 is insensitive to external electricnoise. Therefore, it becomes possible that the angular rate sensor 1 hashigh sensitivity since the electric signal level is increased due to anelectronic noise reduction effect of the conduction film. The conductionfilm is capable for making of one of impurity doped poly-silicon,aluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy.

Further, as shown in FIG. 38, in the angular rate sensor having aconductive film 60 formed on the piezoelectric substrate 5 and anopening over the predetermined region 60 below which the perturbationmasses 6 is formed, the piezoelectric material composing thepiezoelectric film 5 is expelled from the openings. The opening isformed using photolithography technique.

Modification of Fifth to Eighth Embodiments

Further, in the angular rate sensors according to the fifth to eighthembodiments, the driving IDTs and detecting IDTs consists of thecomb-shaped electrode 7-10, 13-16. However, the driving IDTs anddetecting IDTs are limited to these configurations but other shapedelectrodes can be used for the components of the driving IDTs and thedetecting IDTs.

Further, in the angular rate sensors according to the fifth to eighthembodiment, the driving electrodes 7-10 are located such that thedriving electrodes 7-8 are disposed at one side of the predeterminedregion 60 and the driving electrodes 9-10 are disposed at another sideof the predetermined region 60. However, it is not necessary to form thedriving electrodes 7-10 at both sides of the predetermined region 60. Ina modification of the angular rate sensor according to the first tothird embodiments, at least one pairs of the driving electrodes 7-8 andthe driving electrodes 9-10 is formed.

Further, it is preferable that in the method for manufacturing theangular rate sensor 1 according to the sixth embodiment, a selectiveetching technique having the proper etching ratio of one material of thedriving electrode 7-10 to another material of the perturbation masses 6is used. If it is not possible to use the selective etching technique,the liftoff technique is applicable for forming the perturbation masses6.

Ninth Embodiment

Referring to FIGS. 39A to 44B, an angular rate sensor according to theninth embodiment of the present invention will now be explained. In thisembodiment, the identical components in structure to those in theprevious embodiments are assigned to the same reference numerals forsimplicity and the description of these components are omitted from thedetailed explanations.

FIG. 39A and FIG. 39B show a structure of an angular rate sensor 1according to a ninth embodiment of the present invention. FIG. 39A is abird's-eye view of the angular rate sensor 1 and FIG. 39B is across-sectional view thereof taken along line A-A in FIG. 39A.

As shown in FIG. 39B, the angular rate sensor 1 according to a ninthembodiment comprising a supporting member 2 made of an insulatingsubstrate or a semiconductor substrate having a high resistivity. Theangular rate sensor 1 further comprises sensing components disposed onthe upper surface of the supporting member 2.

A thin insulator film 3 is formed on the upper surface of thesemiconductor substrate 3 so as to cover a whole surface of thesemiconductor substrate 3. The thin insulator film is made of siliconoxide (SiO₂), for example. The thickness of the thin insulator film isorder of 1 μm.

On the thin insulator film 3, a first electrode 4 which is severalhundreds nanometer is formed. The first electrode 4 is composed of oneof aluminum (Al), aluminum (Al)-silicon (Si) alloy, aluminum(Al)-silicon (Si)-copper (Cu) alloy, and impurity-doped poly-siliconwith the semiconductor production technology for very-large-scaleintegrated circuits (VLSIs) by taking account of an environmental metalpollution.

A piezoelectric film 5 is disposed on the first electrode 4 so as tocover the whole surface of the first electrode 4. The thickness of thepiezoelectric film 5 is about several micrometers. For example, thepiezoelectric film 5 is made of aluminum nitride (AlN) or zinc oxide(ZnO), but also of a piezoelectric and ferroelectric film such asPTZ:Pb(ZrTi)O₃, PT:PbTiO₃, and the like. In the case where thepiezoelectric film is made of aluminum nitride (AlN), an integration ofthe other functional device, such as complementarymetal-oxide-semiconductors (CMOS), into the angular rate sensor ispossible to achieve without taking into account of an environmentalmetal pollution.

A thin insulator film 20 is formed on the piezoelectric film 5. It isespecially preferable in the case where the piezoelectric film 5 is madeof AlN that the thin insulator film 20 is formed on the supper surfaceof the piezoelectric film 5 in order to suppress a leakage electriccurrent flowing along the z-direction in the piezoelectric film 5.

Further, perturbation masses 6 also serving as to second electrode, anddetecting electrodes 7-10 are disposed on the piezoelectric film 5. Aplurality of the detecting electrodes constructs a detectinginter-digital transducers (hereinafter, “detecting IDTs”)

If the perturbation masses 6 also serving as the second electrode anddetecting electrodes 7-10 are made of any one of impurity dopedpoly-silicon, aluminum (Al), and Al—Si—Cu with the semiconductorproduction technology for VLSIs, it is not necessary to take intoaccount of the environmental metal pollution during manufacturing theangular rate sensor.

Further, if the perturbation masses 6 also serving as the secondelectrode and detecting electrodes 7-10 are made of one of aluminum(Al), platinum (Pt), tungsten (W), and rubidium (Ru), mass density ofthe first electrodes is increased so that total weight of the firstelectrodes is increased.

As shown in FIG. 39A, the perturbation mass also serving the secondelectrodes 6 is formed in shape of rectangular. The detecting electrodes7-10 are divided into the first group including the detecting electrodes7-8 of which a first IDT consists, and the second group including thedetecting electrodes 9-10 of which a second IDT consists. The secondelectrodes 7-10 formed in a comb-shape. The first IDT and the second IDTare positioned such that the perturbation mass also serving the secondelectrodes 6 is sandwiched between the first IDT and the second IDT in adirection perpendicular to the longer edge of the rectangular shapedperturbation mass 6.

As shown in FIG. 39B, in the angular rate sensor 1 according to thisembodiment, the electric power supply 12 connects to the perturbationmass also serving the second electrodes 6 and the first electrode 3 viathe supplying driving voltage line 11 and the ground line 13,respectively. For example, an alternating current (AC) voltage +B[V] issupplied to the perturbation mass also serving the second electrodes 6,whereas an alternating current (AC) voltage −B[V] is applied to thefirst electrode 4. The perturbation mass also serving the secondelectrodes 6 is excited to vibrate in the z-direction by the alternatingcurrent (AC) voltage.

The driving electrodes 7-8 are used to excite a first elastic acousticwave in order to cause a standing wave on which a Coriolis force actswhen the angular rate sensor is rotated. The driving electrodes 7, 8 arecoupled for constituting a first driving inter-digital transducer(driving IDT). Each driving electrodes 7, 8 is formed in a comb-shape,that is, the driving electrodes 7, 8 include tooth components 7 a, 8 a,which are disposed to be parallel to the y-axis and connectingcomponents 7 b, 8 b, which are disposed to be perpendicular to they-direction and serves as connectors of a plurality of the toothcomponents 7 a, 8 a. In the driving electrodes 7, 8, the toothcomponents 7 a, 8 a, are arranged to be interlaid with each other. Theconnecting components 7 b, 8 b, are located face to face across thetooth components 7 a, 8 a. A spacing of tooth components 7 a, 8 a, ofthe driving electrodes 7, 8, in the x-direction determines a wavelengthof an elastic acoustic wave generated by the perturbation mass 6,themselves.

Similarly to the driving electrodes 7, 8, the driving electrodes 9, 10are coupled for constituting a second driving inter-digital transducer(driving IDT). The second driving IDT composed of detecting electrodes9, 10, is positioned on an opposite side of the predetermined region 60to the first driving IDT composed of the detecting electrodes 7, 8. Eachdriving electrodes 9, 10 is formed in a comb-shape. The drivingelectrodes 9, 10 include tooth components 9 a, 10 a, which are disposedto be parallel to the y-axis and connecting components 9 b, 10 b, whichare perpendicular to the y-direction. The connecting components 9 b, 10b bridge the tooth components 9 a, 10 a, thereby. As is the case of thedriving electrodes 7, 8, the tooth components 9 a, 10 a, are arranged tobe interlaid with each other and located to be spaced apart with aperiodicity of one half the wavelength of the elastic acoustic wavegenerated by the driving IDTs 7-10 themselves.

The driving electrodes 7-10 are made of one of impurity dopedpoly-silicon, aluminum (Al), aluminum-base alloy, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy. If the driving electrodes 7-10 are made of anyone of impurity doped poly-silicon, aluminum (Al), and Al—Si—Cu with thesemiconductor production technology for VLSIs, it is not necessary totake into account of the environmental metal pollution duringmanufacturing the angular rate sensor. If the perturbation mass alsoserving the second electrodes 6 is made of metals or metallic alloysincluding platinum (Pt), tungsten (W), and gold (Au). In such aconfiguration of the perturbation masses 6, the Coriolis force acting onthe perturbation masses 6 is emphasized so as to increase thedisplacement of the perturbation masses 6. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.

In the following, the operations of the angular rate sensor 1 accordingto the ninth embodiment of the present invention will be described.

In the angular rate sensor 1 according to this embodiment, the sensingunit is driven by applying a driving voltage to the perturbation masses6. For example, an alternating current (AC) voltage +B[V] is supplied tothe perturbation masses 6, whereas an alternating current (AC) voltage−B[V] is applied to the first electrodes 3 from the electric powersupply 12 via the supplying driving voltage line 11 and the ground line13. So that, a voltage difference between the perturbation mass 6 andthe first electrodes 4 is generated. By the piezoelectric effect, theperturbation mass 6 is vibrated in the z-direction so that elasticacoustic waves are caused and propagated along the x-direction.

If each perturbation mass 6 has a mass, m, and a velocity of eachperturbation mass 6

, when the angular rate sensor 1 undergoes a rotary motion with anangular rotation

perpendicular to the direction of the velocity of each perturbation mass6

causes the Coriolis force

perpendicular to the directions of both the vibrating motion

and the angular rotation

. Where the velocity of each perturbation mass 6,

, and the angular rotation

. That is, if the angular rate sensor 1 rotates about the y-direction inFIGS. 31A and 31B, the Coriolis force is generated in the x-directionwith amplitude |

|=2m

×

|. This Coriolis force causes a propagating elastic acoustic wave in thex-direction. This propagating elastic acoustic wave arrives at thedetecting electrodes 7-10 and generates a change in electric potentialof the detecting electrodes 7-10. Therefore, the angular rotation

can be obtained.

In the angular rate sensor 1 having the above mentioned arrangement,rotation rate is measured based on the difference between a measuredelectric power detected by the comb-shaped detecting electrodes 7-8 andthat by the comb-shaped detecting electrodes 9-10 so that effects ofexternal noise in elastic acoustic waves are removed from final resultsof the measurement. The reason of this is if one of the output voltagesdetected by the detecting electrodes 7-8 and by the detecting electrodes9-10, the measured voltage sometimes contains external noises. However,using the difference between a measured electric power detected by thecomb-shaped detecting electrodes 7-8 and that by the comb-shapeddetecting electrodes 9-10 leads a cancellation of the external noisesduring the measurement.

In the following, the operations of the angular rate sensor 1 accordingto the first embodiment of the present invention will be described.

The method for manufacturing the angular rate sensor 1 according to thisembodiment includes a preparing step for preparing the semiconductorsubstrate 2, a first electrode forming step for depositing the firstelectrode 4 on the semiconductor substrate 2 at least over a regionabove which perturbation masses 6 is formed, a piezoelectric filmforming step for depositing the piezoelectric film 5 on at least one ofthe first electrode 3 and the semiconductor substrate 2, an insulatorfilm forming step for forming the insulator film 20 on the piezoelectricfilm 5, and a fabricating step for fabricating a plurality of featureson the piezoelectric film 5, wherein the plurality of features includesthe perturbation masses also serving the driving electrode 6 for causinga first elastic acoustic wave in the piezoelectric film 5, a detectingelectrode 7-10 for detecting a second elastic acoustic wave caused bythe Coriolis force acting on the first elastic acoustic wave.

In more detail, first, a (100)-oriented silicon substrate 2 of 400 μmthickness is prepared. In this arrangement, an integration of the otherfunctional device, such as complementary metal-oxide-semiconductors(CMOS), into the angular rate sensor is possible to achieve without anyfuture technology.

Then, a thin film of metal or conductive metallic alloy includingaluminum (Al), aluminum-base alloy, Al—Si—Cu, titanium (Ti),titanium-base alloy, tungsten, tungsten-base alloy, molybdenum, andmolybdenum-base alloy, is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid to form the first electrode4. The thickness of the first electrodes 4 is order of 1.0 to 1.5 μm.Sputtering technique is also applicable to perform this step.

A piezoelectric film 5 is disposed on the first electrode 4 so as tocover the whole surface of the first electrode 4. The thickness of thepiezoelectric film 5 is about several micrometers. For example, thepiezoelectric film 5 is made of aluminum nitride (AlN) or zinc oxide(ZnO), but also of a piezoelectric and ferroelectric film such asPTZ:Pb(ZrTi)O₃, PT:PbTiO₃, and the like. In the case where thepiezoelectric film is made of aluminum nitride (AlN), an integration ofthe other functional device, such as complementarymetal-oxide-semiconductors (CMOS), into the angular rate sensor ispossible to achieve without taking into account of an environmentalmetal pollution.

Further, perturbation masses 6 also serving as a second electrode, anddetecting electrodes 7-10 are disposed on the piezoelectric film 5. Aplurality of the detecting electrodes constructs a detectinginter-digital transducers (hereinafter, “detecting IDTs”)

If the perturbation masses 6 also serving as a second electrode anddetecting electrodes 7-10 are made of any one of impurity dopedpoly-silicon, aluminum (Al), and Al—Si—Cu with the semiconductorproduction technology for VLSIs, it is not necessary to take intoaccount of the environmental metal pollution during manufacturing theangular rate sensor.

Next, a thin film of metal or conductive metallic alloy includingaluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy, isdeposited using an e-beam evaporator, and patterned by a phosphoric(H₃PO₄)-based acid to form the driving electrodes 7-10 on an uppersurface of the piezoelectric film 5, in the same way shown in FIG. 4C.

Finally, the supplying driving voltage lines 11 and ground lines 12 areformed and connected to the driving electrodes 7-10 by wire bonding.

In the angular rate sensor 1 having the second electrode 6 configured toserve as the perturbation mass, the second electrode 6 vibrate in az-direction defined as a perpendicular direction to the upper surface ofthe piezoelectric substrate 5 due to the piezoelectric effect ifalternative current (AC) voltage is applied between the first and secondelectrodes 3, 6. Thus, it becomes possible that information of rotationof the piezoelectric substrate 5 is obtained via the vibration of thesecond electrode 6 formed on the upper surface of the piezoelectricsubstrate 5. On of advantages of the angular rate sensor 1 of the abovedescribed type is that a necessary area for a perturbation mass 6becomes to be smaller than that for metallic dots according to theprevious embodiments serving as perturbation masses on which theCoriolis force acts when an elastic acoustic wave is excited along aparallel direction to the upper surface of the piezoelectric substratesince elastic acoustic wave is generated in the z-direction, i.e., inthe perpendicular direction to the upper surface of the piezoelectricsubstrate 5. Another advantage of the angular rate sensor 1 of the abovedescribed type is that there is not necessary for a driving electrodeand reflectors. Only detecting electrodes 7-10 are necessary. It is notneeded to arrange the driving electrodes and reflectors such that all ofthe driving electrodes, reflectors, and a region wherein metallic dotsserving as perturbation masses on a straight line such that the regionwherein metallic dots and the driving IDT are sandwiched betweenreflectors. Therefore, downsizing of angular rate sensors andintegrating an angular rate sensor and an external driving circuitthereof into a small size integrated device are simultaneously achieved.

In the angular rate sensor 1 according to this embodiment, the drivingAC voltage is directly applied to the perturbation mass 6 which alsoserves as the second electrode so as to generate vibration of theperturbation mass 6 in the z-direction. Therefore, higher resonantfrequency is achieved so that higher efficiency of causing elasticacoustic wave is obtained. In more detail, in the angular rate sensorhaving perturbation mass, driving electrodes, and reflectors whereindriving electrodes and perturbation masses are separately fabricated, astanding wave is generated by the driving electrodes and is rectified toa standing wave by reflectors. Thus, velocity of the perturbation massundergoing oscillatory motion is not large and amplitude of thevibration of the perturbation mass in the z-direction is not so large.An advantage of the present embodiment is that since the secondelectrode 6 is directly driven by an external driving circuit,limitations to the velocity and the amplitude of the second electrodeare removed. Since the Coriolis force is proportional to the particlevelocity, the measured voltage generated by an elastic acoustic wavecaused by the Coriolis force due to the piezoelectric effect detected bythe detecting electrodes 7-10 is increased. In consequence, it ispossible to realize an angular rate sensor having high sensitivity.

Especially, in the angular rate sensor according to this embodiment, alarger and heavy electrode 6 can be arranged. Since the Coriolis forceis proportional to the particle mass, it is possible to realize anangular rate sensor having high sensitivity.

First Modification of the Ninth Embodiment

Referring to FIG. 40, an angular rate sensor according to the firstmodification of the ninth embodiment will now be explained. In thismodification of the ninth embodiment, the identical components instructure to those in the previous embodiments are assigned to the samereference numerals for simplicity and the description of thesecomponents are omitted from the detailed explanations.

FIG. 40 shows a bird's-eye view of the angular rate sensor 1 accordingto the first modification of the ninth embodiment. The angular ratesensor 1 has the second electrode 6 configured to serve as theperturbation mass, the detecting IDT further comprises a third detectingIDT and a forth detecting IDT. Each of the third and the forth IDTs arecomposed of a plurality of electrodes, 21-22 and 23-24, respectively.The third and the forth IDT is located such that the second electrodeserving as the perturbation mass is sandwiched therebetween and thefirst and the second IDTs are orthogonally arranged on the upper surfaceof the piezoelectric substrate.

This configuration of the first, second, third, and forth detecting IDTsleads to the possibility of detecting rotation rate about multipleorthogonal axes. Thus, this configuration is capable of reducing effectsof direct elastic acoustic waves generated by applying external ACvoltages on measuring voltage relating to elastic acoustic wavesgenerated by the Coriolis force.

Further, the above configuration of the first, second, third, and forthdetecting IDTs is capable of reducing an unnecessary contribution fromthe direct elastic acoustic waves which are not experienced of thescattering by the Coriolis force if angular rate is obtained by at leastof a differences between output voltages between the first and seconddetecting IDTs and a differences between output voltages between thethird and forth detecting IDTs.

Second Modification of the Ninth Embodiment

Referring to FIGS. 41A and 41B, an angular rate sensor according to thesecond modification of the ninth embodiment will now be explained.

The angular rate sensor according to the second modification of theninth embodiment has a different structure of the first electrode 4 andthe thin insulator film 20 relative to those of the ninth embodiment.

FIG. 41A and FIG. 41B show an angular rate sensor 1 according to thisembodiment. FIG. 41A shows a bird's-eye view of the angular rate sensor1 and FIG. 41B shows a cross-sectional view taken along B-B line in FIG.41A.

As shown in FIGS. 41A and 41B, the angular rate sensor 1 has the firstelectrode 4 and the thin insulator film 20, both of which are onlyformed below a region 60 on which the perturbation mass 6 is disposed.

In this arrangement of the first electrode 4 and the thin insulator film20, the weight of the thin insulator film contributes that of theperturbation mass 6 so that the Coriolis force proportional to theparticle mass is increased. Therefore, the angular rate sensor 1 obtainshigh sensitivity.

In more detail, in the case where the thin insulator film 20 is formedover the whole area where the piezoelectric film 5 is formed, theperturbation mass 6 which is configured to serve as the second electrodeand the thin insulator film are simultaneously vibrate in thez-direction when the driving AC voltage is applied between the secondelectrode 6 and the first electrode 3. While the thin insulator film 20is vibrating with the perturbation mass6, the inner stretching stress isgenerated in the thin insulator film 20. From this reason, the weight ofthe thin insulator film 20 does not effectively contribute to the weightof the perturbation mass 6. In contrast to the former case, if the firstelectrode 4 and the thin insulator film 20, both of which are onlyformed below a region 60 on which the perturbation mass 6 is disposed,the inner stretching stress generated in the thin insulator film 20 issuppressed. Therefore, the effective weight of the perturbation mass 6is increased by the weight of the thin insulator film 20.

Further, if the first electrode 4 is only formed over the predeterminedregion 60 on which the perturbation mass 6 is formed, the firstelectrode is not disposed over a region above which the drivingelectrodes 7-10 is formed. This arrangement of the first electrode 4 andthe thin insulator film 20 enables to reduce a electric current leakagebetween the first and second electrodes.

The method for manufacturing the angular rate sensor 1 according to thisembodiment includes a preparing step for preparing the semiconductorsubstrate 2, a opening forming step for forming the opening in thesemiconductor substrate 2, a first electrode forming step for disposingthe first electrode 4 in the opening, a piezoelectric film forming stepfor forming the piezoelectric film 5 so as to cover the opening and thesemiconductor substrate 2, a fabricating step for fabricating a drivingelectrode 7-10 on the piezoelectric film 5, a insulator film formingstep for disposing the insulator film 20 on the piezoelectric film 5,and a perturbation mass forming step for forming the perturbation masses6 on the thin insulator film 20.

First, in the method for manufacturing the angular rate sensor 1, a(100)-oriented silicon substrate 2 of 400 μm thickness is prepared.

Next, the opening for which the first electrode 4 is accommodated, isformed in the semiconductor substrate 2 such as silicon substrate usingphotolithography technique. For example, if silicon substrate is used asthe semiconductor substrate, an upper surface of the silicon substrateis cleaned by using acetone, isopropanol and trichloroethylene, in turn.The surface of the silicon substrate is then thoroughly rinsed inde-ionized water and subsequently heated on a hot plate to removesurface moisture. Upon cooling the substrate on a heat sinking plate,photoresist is then spin coated on the silicon substrate after soakingthe hexamethyldisilazane, an adhesion agent. Then the soft-bake processis performed in which the substrate is heated. A negative-mask is set inplace over the photoresist. The negative mask is a template defining thepatterns of the substrate. The silicon substrate is then exposed toultra-violet (UV) light such that the regions of the resist which areexposed become soluble to the developer. Thus, a pattern is formedhaving a plurality of apertures therethrough. The substrate is soaked inphotoresist and developed until the sections which has been exposed toUV light, and are therefore soluble, are etched away.

For example, dry etching is applicable to etch the opening. A gasincluding fluorine, such as a CF-type gas including C4F8, or a SF-typegas including SF6, is used as etching gas for the dry etching. Theetching gas is changed into plasma to produce fluorine radicals, andetching is performed by processing the silicon substrate with thefluorine radicals.

Next, a metal or conductive metallic alloy 40 including platinum (Pt),gold (Au), and tungsten (W) is deposited using an e-beam evaporator, andpatterned by a phosphoric (H₃PO₄)-based acid on the exposed surface ofthe opening and upper surface of the silicon substrate 2 to form thefirst electrode 4.

Next, a several micrometer thick film of a piezoelectric film such asaluminum nitride (AlN), zinc oxide (ZnO), zirconate titante (PZT), leadtitanate (PT), lithium tantalite (LiTaO₃), and lithium tantalite (LT),is disposed so as to cover the whole region of an upper surface of thefirst electrode 4. If an aluminum nitride (AlN) film is used as thefirst electrode 4, the AlN film is patterned by a tetra methyl ammoniumhydroxide-based solvent.

Next, a thin film of metal or conductive metallic alloy includingaluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy, isdeposited using an e-beam evaporator, and patterned by a phosphoric(H₃PO₄)-based acid to form the driving electrodes 7-10 on an uppersurface of the piezoelectric film 5, in the same way shown in FIG. 4C.

Next, a thin silicon dioxide film which is thinner than 1 μm is grown onthe piezoelectric film 5. The silicon dioxide film is formed, forexample by sputtering.

Next, a thin film of metal or conductive metallic alloy includingaluminum (Al), aluminum-base alloy, titanium (Ti), titanium-base alloy,tungsten, tungsten-base alloy, molybdenum, and molybdenum-base alloy, isdeposited using an e-beam evaporator, and patterned by a phosphoric(H₃PO₄)-based acid to form the perturbation mass 6 on an upper surfaceof the thin insulator film 20.

Third Modification of the Ninth Embodiment

Referring to FIGS. 42A and 42B, an angular rate sensor according to thethird modification of the ninth embodiment will now be explained.

The angular rate sensor according to the third modification of the ninthembodiment has a via hole 2 a formed from the semiconductor substrate 2so as to obtain a structure in which the lower surface of the firstelectrode is exposed to the air.

FIG. 42A and FIG. 42B show an angular rate sensor 1 according to thisembodiment. FIG. 42A shows a bird's-eye view of the angular rate sensor1 and FIG. 42B shows a cross-sectional view 30, taken along C-C line inFIG. 42A.

As shown in FIGS. 42A and 42B, the angular rate sensor has the via holeformed from the semiconductor substrate 2 over a region below thepredetermined region 60 on which the perturbation mass 6 is formed so asto obtain a structure in which the lower surface of the first electrodeis exposed to the air.

In this configuration, an unnecessary vibration of the perturbation massis generated if the via hole has an excess size. Thus, it is preferablethat a length of the via hole along a x-direction, that is defined by adirection in which the first and second detecting IDTs are in alignmentwith each other, is shorter than a spacing between the first and secondIDTs and the ends of the perturbation mass lie off the via hole alongthe x-direction, so that the unnecessary vibration of the perturbationmass is suppressed.

In contrary to the case in the x-direction, a longer length of the viahole overlapping edges of the first and second IDTs along they-direction, which is defined by a direction orthogonal to thex-direction on the surface of the thin piezoelectric film, ispreferable. In such a configuration of the via hole, the first andsecond IDTs becomes to be possible to output a higher lever of electricsignals proportional to the Coriolis force acting on the perturbationmass when the thin piezoelectric film rotates about the x-axis. Thus, itis preferable that the ends of the via hole lie off the perturbationmass along the y-direction, so that the unnecessary vibration of theperturbation mass is suppressed.

Dry etching is applicable to etch the via hole 2 a. A gas includingfluorine, such as a CF-type gas including C4F8, or a SF-type gasincluding SF6, is used as etching gas for the dry etching. The etchinggas is changed into plasma to produce fluorine radicals, and etching isperformed by processing the silicon substrate with the fluorineradicals.

Forth Modification of the Ninth Embodiment

Referring to FIG. 43, an angular rate sensor according to the forthmodification of the ninth embodiment will now be explained.

As shown in FIG. 43, the angular rate sensor 1 is arranged to have thefirst, second, third, and forth detecting IDTs so as to detect rotationrate about multiple orthogonal axes, the ends of the perturbation massboth along the x-direction and the y-direction preferably lie off thevia hole. In this arrangement, the unnecessary vibration of theperturbation mass is suppressed.

The second electrode 6 also serving as the perturbation mass preferablyconsists of a single electrode so that a mass density of a region wherethe second electrode is formed is increased. Therefore, still preferablythe second electrode is formed in rectangular shaped in order toincrease the mass density of the region where the second electrode isformed. The fact that the mass density of the region where the secondelectrode is formed is large brings the second electrode to vibrate withlarge amplitude in a z-direction defined as a piezoelectric filmthickness direction. When the thin piezoelectric film rotate, electriccurrent relating to the Coriolis force acting on the second electrodevibrating along the z-direction whose amplitude is proportional tovibrating velocity thereof is generated. Therefore, the larger theamplitude of the vibrating velocity of the second electrode is, thehigher the sensitivity of the angular rate sensor is, since the secondelectrode vibrates in the z-direction with a larger amplitude.

In a modification of the angular rate sensor of this type, a firstelectrode is formed on the lower surface of the thin piezoelectric filmin a region above which the second electrode is disposed.

In this configuration, the first electrode is only formed on the otherregion above which the detecting IDTs are disposed. Therefore, a regionbelow the detecting IDTs in the thin piezoelectric film escaped from anexistence of electric fields and elastic acoustic waves because ofnonexistence of the first electrode below the detecting IDTs.

In a further modification of the angular rate sensor having the secondelectrode also serving as the perturbation mass, an thin insulator filmis formed so as to cover the upper surface of the thin piezoelectricfilm on which the second electrode is disposed.

Preferably, the thin insulator film is disposed on the upper surface ofthe thin piezoelectric film over a region on which the second electrodealso serving as the perturbation mass is disposed. This arrangement ofthe thin insulator film enables to reduce a electric current leakagebetween the first and second electrodes. There is a further advantage ofthe angular rate sensor of this type where weight of a part of the thininsulator film located below the second electrode contribute to theperturbation mass in addition to the weight of the second electrode sothat a high sensitivity of the angular rate sensor is achieved.

Still further, the angular rate sensor according to the presentinvention, the piezoelectric substrate and the thin piezoelectric filmis made of one of aluminum nitride (AlN), zinc oxide (ZnO), zirconatetitante (PZT), lead titanate (PT), lithium tantalite (LiTaO₃), andlithium tantalite (LT). If the thin piezoelectric film is made of AlN,an integration of the other functional device, such as complementarymetal-oxide-semiconductors (CMOS), into the angular rate sensor ispossible to achieve without taking account of an environmental metalpollution.

Still further, the angular rate sensor according to the presentinvention, if at least one of the first electrode or the secondelectrode also serving as the perturbation mass is made of one ofaluminum (Al), aluminum (Al)-silicon (Si) alloy, aluminum (Al)-silicon(Si)-copper (Cu) alloy, and impurity-doped poly-silicon, the firstelectrode is possible to be formed by a semiconductor productionprocessing technology with contributing prevention of environmentalmetal pollution.

Still further, the angular rate sensor according to the presentinvention, at least one of the first electrode and the second electrodealso serving as the perturbation mass is made of one of aluminum (Al),platinum (Pt), tungsten (W), and rubidium (Ru), mass density of thefirst and second electrodes is increased so that total weight of thefirst and second electrodes is grown.

Fifth Modification of the Ninth Embodiment

Referring to FIG. 44A and FIG. 44B, an angular rate sensor according tothe fifth modification of the ninth embodiment will now be explained.

The angular rate sensor according to the present invention, it ispreferable that the second electrode is composed of a plurality ofmetallic island films which are connected electrically each others andare driven simultaneously by an external electric supply.

FIG. 44A shows a bird's-eye view of the angular rate sensor 1 and FIG.44B shows a top-view of the predetermined region 60 on which the secondelectrodes 6 are formed. In the angular rate sensor 1 according to thisembodiment, the second electrodes 6 is composed of a plurality ofmetallic island films which are located at lattice points of a regularlattice. The regular lattice is, for example, 3×3 square lattice havingnine lattice points expanded on the x-y plane. Each second electrode 6are supplied driving voltage from the electric power supply 12 via thesupplying driving voltage lines 11. Each second electrode 6 start tovibrate in the z-direction due to the piezoelectric effect coherentlysince all second electrode 6 are connected each other. In order toamplify an elastic acoustic wave caused at the every lattice point wherethe second electrode 6 is formed, the spacing between the neighboringelectrodes 6 is fixed by the integer multiple of the wave length of theelastic acoustic wave.

In this arrangement of the angular rate sensor, elastic acoustic wavesgenerated at individual electrodes composing the second electrode by theCoriolis force are synchronously emphasized. Therefore, high sensitivityof the angular rate sensor is achieved.

Still further, an angular rate sensing device is provided by integratinga plurality of an angular rate sensors into a single device such that afinal result of measured angular rate is obtained based on electricsignals outputted from the plurality of the angular rate sensors.Therefore, an angular rate sensing device producing an accurate measuredresult and having high sensitivity is provided.

1-66. (canceled)
 67. An angular rate sensor, of which operations are ina three-dimensional coordinate system consisting of x, y, andz-orthogonal directions, based on a first and a second elastic acousticwave propagating generated in an elastic material along the x andy-directions, respectively, in an elastic material on which a Coriolisforce acts in response to a rotary motion of the angular rate sensorabout the x-direction, comprising: a piezoelectric film for serving asthe elastic material having a first and a second surfaces with an x-yplane created by the x and y directions in the coordinate system, thepiezoelectric film being composed of piezoelectric material; a firstelectrode disposed on the first surface of the piezoelectric film, thefirst electrode being made of conductive material including a metal anda metallic alloy; a second electrode disposed on the second surface ofthe piezoelectric film so as to cover at least a predetermined regionbeing opposite to the first electrode and configured to also serve aperturbation mass vibrating in the z-direction perpendicular to the x-yplane, the second electrode being made of conductive material includinga metal and a metallic alloy in order to drive in the piezoelectric filma first elastic acoustic wave, wherein the second electrode vibratecoherently with first elastic wave, the driving transducer being inalignment with the predetermined area on which the second electrode isformed along the x-direction so that the second electrode is displacedby the Coriolis force for generating a second elastic acoustic wavealong the y-direction; and a detecting transducer disposed on the firstsurface of the piezoelectric film comprised of piezoelectric materialsuch that the predetermined area on which the second electrode is formedis adjoin to the detecting transducer and is in alignment along they-direction, for detecting the second elastic acoustic wave, and forproviding an output indicative of the second elastic acoustic wavegenerated by the Coriolis force proportional to an angular rate of therotary motion of the angular rate sensor itself.
 68. The angular ratesensor according to claim 67, wherein the detecting transducer iscomposed of a first and second detecting transducers located such thatthe second electrode is sandwiched between the first and seconddetecting transducers along the x-direction.
 69. The angular rate sensoraccording to claim 68, wherein the output indicative of the secondelastic acoustic wave is estimated based on a difference of electricpotentials created by the first and second detecting transducers due toa piezoelectric effect by which a mechanical deformation energy convertsinto electric potential.
 70. The angular rate sensor according to claim69, wherein the detecting transducer is composed of a first, second,third and forth detecting transducers located such that the secondelectrode is sandwiched between the first and second detectingtransducers along the x-direction and between the third and forthdetecting transducers along the y-direction.
 71. The angular rate sensoraccording to claim 70, wherein the output indicative of the secondelastic acoustic wave is estimated based on a first difference ofelectric potentials created by the first and second detectingtransducers and a second difference of electric potentials created bythe third and forth detecting transducers due to a piezoelectric effectby which a mechanical deformation energy converts into electricpotential.
 72. The angular rate sensor according to claim 67, furthercomprising: a supporting member having a surface on which thepiezoelectric film is disposed such that the first surface of thepiezoelectric film is connected with the surface of the supportingmember.
 73. The angular rate sensor according to claim 71, furthercomprising: a supporting member having a surface on which thepiezoelectric film is disposed such that the first surface of thepiezoelectric film is connected with the surface of the supportingmember.